Method for manufacturing a surface acoustic wave

ABSTRACT

In a manufacturing method for a SAW apparatus a first insulating layer is formed on the entire surface of a piezoelectric LiTaO 3  substrate. By using a resist pattern used for forming an IDT electrode, the first insulating layer in which the IDT electrode is to be formed is removed. An electrode film made of a metal having a density higher than Al or an alloy primarily including such a metal is disposed in the area in which the first insulating layer is removed so as to form the IDT electrode. The resist pattern remaining on the first insulating layer is removed. A second insulating layer is formed to cover the first insulating layer and the IDT electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave (SAW) apparatusused in, for example, resonators and bandpass filters and also to amanufacturing method for this type of SAW apparatus. More specifically,the invention relates to a SAW apparatus having a structure in which aninsulating layer is disposed to cover an interdigital (IDT) electrodeand also to a manufacturing method for this type of SAW apparatus.

2. Description of the Related Art

DPX or RF filters used in mobile communication systems need to satisfywide-band and good temperature characteristics. In known SAW apparatusesused in DPX or RF filters, piezoelectric substrates formed of36°-50°-rotated Y-plate X-propagating LiTaO₃ are used. This type ofpiezoelectric substrate has a temperature coefficient of frequency (TCF)of about −40 to −30 ppm/° C. In order to improve the temperaturecharacteristic, it is known that a SiO₂ film having a positive TCF isformed to cover an IDT electrode on a piezoelectric substrate. Anexample of a manufacturing method for this type of SAW apparatus isshown in FIGS. 109A through 109D.

As shown in FIG. 109A, a resist pattern 52 is formed on a piezoelectricsubstrate 51 except for an area in which an IDT electrode is to beformed. Then, as shown in FIG. 109B, an electrode film 53, which servesas an IDT electrode, is formed on the entire surface of thepiezoelectric substrate 51. Subsequently, by using a resist stripper,the resist pattern 52 and a metallic film attached to the resist pattern52 are removed, thereby forming an IDT electrode 53A, as shown in FIG.109C. Then, as shown in FIG. 109D, a SiO₂ film 54 is formed to cover theIDT electrode 53A.

For achieving an object other than the improvement of the TCF, anothermanufacturing method for a SAW apparatus in which an insulating ornon-conductive protective film is formed to cover an IDT electrode isdisclosed in Japanese Unexamined Patent Application Publication No.11-186866. FIG. 110 is a schematic sectional view illustrating a SAWapparatus 61 taught in JP 11-186866. In the SAW apparatus 61, an IDTelectrode 63 made of Al or an alloy primarily including Al is disposedon an insulating substrate 62. In an area other than an area in whichthe IDT electrode 63 is disposed, an insulating or non-conductiveinter-electrode-finger film 64 is disposed. An insulating ornon-conductive protective film 65 is also disposed to cover the IDTelectrode 63 and the inter-electrode-finger film 64. In the SAWapparatus 61 disclosed in JP 11-186866, the inter-electrode-finger film64 and the protective film 65 are made of an insulating material, forexample, SiO₂, or a non-conductive material, for example, silicone. Byforming the inter-electrode-finger film 64, discharging between theelectrode fingers caused by a pyroelectric property unique to thepiezoelectric substrate 62 can be suppressed.

Japanese Unexamined Patent Application Publication No. 61-136312 teachesthe following type of one-port SAW resonator. An electrode made of ametal, such as aluminum or gold, is disposed on a piezoelectricsubstrate made of quartz or lithium niobate. Then, after a SiO₂ film isformed, it is planarized. In this type of resonator, good resonancecharacteristics can be achieved by planarizing the SiO₂ film.

As shown in FIGS. 109A through 109D, in the manufacturing method for SAWapparatuses in which the SiO₂ film 54 is formed for improving the TCF,the height of the SiO₂ film 54 is different between a portion with theIDT electrode 53A and a portion without the IDT electrode 53A. Becauseof the differences in the height of the SiO₂ film 54, the insertion lossis increased. These differences in height increase as the thickness ofthe IDT electrode 53A becomes larger. Thus, the thickness of the IDTelectrode 53A cannot be increased.

In the SAW apparatus 61 taught in JP 11-186866, after theinter-electrode-finger film 64 is formed between the electrode fingersof the IDT electrode 63, the protective film 65 is formed. Accordingly,the height of the protective film 65 is uniform, unlike the SAWapparatus shown in FIGS. 109A through 109D.

In this configuration, because the inter-electrode-finger film 64 isformed in contact with the IDT electrode 63 which is made of Al or analloy primarily including Al, a sufficient reflection coefficient is notobtained in the IDT electrode 63, thereby causing the generation ofripples in the resonance characteristics.

Also, in the manufacturing method taught in JP 11-186866,the resistformed on the inter-electrode-finger film 64 must be removed by a resiststripper before forming the protective film 65. In this case, the IDTelectrode 63 may be disadvantageously eroded by the resist stripper.This requires the use of erosion-resistant metal for the IDT electrode63, thereby decreasing flexibility in selecting the type of metal usedin the IDT electrode 63.

In the one-port SAW resonator taught in JP 61-136312, quartz or lithiumniobate is used for the piezoelectric substrate, and the electrode ismade of aluminum or gold. In JP 61-136312, only the embodiment in whichthe electrode is made of Al and is disposed on a quartz substrate istaught, and no specific reference is made to a SAW apparatus using asubstrate made of another type of material or an electrode made ofanother type of metal.

JP 61-136312 teaches that superior resonance characteristics areachieved by planarizing the SiO₂ film. Then, in order to obtain awide-band filter, the present inventors formed a one-port SAW resonatorhaving a structure similar to the structure taught in JP 61-136312,except that a LiTaO₃ substrate having a large electromechanical couplingcoefficient was used as the piezoelectric substrate. The presentinventors then examined the characteristics of the one-port SAW filter.

More specifically, an Al electrode was formed on the LiTaO₃ substrate,and then, a SiO₂ film was formed and the surface of the SiO₂ film wasplanarized. However, a considerable deterioration in the characteristicsafter the formation of the SiO₂ film was observed, and the presentinventors found that this SAW resonator cannot be put to practical use.

By using a LiTaO₃ substrate or a LiNbO₃ substrate having a largerelectromechanical coupling coefficient than quartz, the fractionalbandwidth is increased considerably. However, the present inventorsfound that, after the formation of an Al electrode on a LiTaO₃ substrateand after the formation of a SiO₂ film, the reflection coefficient wassharply decreased to about 0.02, as shown in FIGS. 2 and 3, caused bythe planarization of the SiO₂ film. FIGS. 2 and 3 illustrate therelationship between the reflection coefficient and the thickness H/λ ofthe IDT electrodes when the IDTs are made of Al, Au, Pt, Cu, and Ag andthe SiO₂ film were formed on a LiTaO₃ substrate having Euler angles (0°,126°, 0°). The solid lines in FIGS. 2 and 3 represent a relationshipbetween the reflection coefficient and the thickness H/λ of the IDTelectrodes when the surface of the SiO₂ film was not planarized. Thebroken lines indicate a relationship between the reflection coefficientand the thickness H/λ of the IDT electrodes when the surface of the SiO₂film was planarized.

FIGS. 2 and 3 show that, when the Al electrode was used, the reflectioncoefficient is decreased considerably to about 0.02 by the planarizationof the surface of the SiO₂ film, regardless of the thickness of the IDTelectrode. Accordingly, a sufficient stop band cannot be achieved,causing the generation of sharp ripples in the vicinity of theantiresonant frequency.

It is known that the reflection coefficient becomes larger as thethickness of an electrode is increased. As is seen from FIGS. 2 and 3,the reflection coefficient is not increased as the thickness of the Alelectrode is increased when the surface of the SiO₂ film was planarized.

In contrast, as is seen from FIG. 2, the reflection coefficient isincreased as the thickness of the Au or Pt electrode is increased evenwhen the surface of the SiO₂ film was planarized.

SUMMARY OF THE INVENTION

Based on the above-described discovery and to overcome the problemsdescribed above, preferred embodiments of the present invention providea SAW apparatus in which an insulating layer is formed between electrodefingers of an IDT electrode and on the IDT electrode in order to achievea sufficiently large reflection coefficient of the IDT electrode and inorder to suppress characteristic deterioration caused by ripplesgenerated in the resonance characteristics, which results in superiorresonance characteristics and superior filter characteristics. Anotherpreferred embodiment of the present invention provides a manufacturingmethod for such a novel SAW apparatus.

Another preferred embodiment of the present invention provides a SAWapparatus that has superior characteristics, for example, a sufficientlylarge reflection coefficient of an IDT electrode and a high degree offlexibility in selecting the type of material forming the IDT electrodeso as to decrease the possibility of erosion of the IDT electrode, and amanufacturing method for such a novel SAW apparatus.

Yet another preferred embodiment of the present invention provides a SAWapparatus that has superior characteristics, for example, a sufficientlylarge reflection coefficient of an IDT electrode, a decreasedpossibility of erosion of the IDT electrode, and a superior temperaturecoefficient of frequency (TCF), and a manufacturing method for such anovel SAW apparatus.

According to a first preferred embodiment of the present invention, aSAW apparatus includes a piezoelectric substrate, at least one electrodedisposed on the piezoelectric substrate and including at least one of ametal having a density higher than Al and an alloy primarily including ametal having a density higher than Al, a first insulating layer providedon the piezoelectric substrate in an area other than an area in whichthe electrode is provided such that the first insulating layer hassubstantially the same thickness as the thickness of the electrode, anda second insulating layer arranged to cover the electrode and the firstinsulating layer. In the first preferred embodiment of the presentinvention, the density of the electrode is about 1.5 times or greaterthan the density of the first insulating layer.

With this configuration, a sufficient reflection coefficient of theelectrode can be obtained. Thus, ripples generated in the resonancecharacteristic or the antiresonance characteristic are shifted outsidethe pass band, and the ripples themselves are suppressed. The TCF isalso improved. Additionally, because the height of the electrode issubstantially the same as that of the first insulating layer, theinsertion loss can be minimized.

According to a second preferred embodiment of the present invention, aSAW apparatus includes a piezoelectric substrate, at least one electrodeprovided on the piezoelectric substrate, a protective metal filmprovided on the electrode and including a metal or an alloy havinghigher erosion-resistant characteristics than the metal or the alloyforming the electrode, a first insulating layer arranged on thepiezoelectric substrate in an area other than an area in which theelectrode is arranged so that the thickness of the first insulatinglayer is substantially the same as the total thickness of the electrodeand the protective metal film, and a second insulating layer provided tocover the protective metal film and the first insulating layer.

With this configuration, because the electrode is covered with theprotective metal film and the first insulating layer, the erosion of theelectrode by a resist stripper when removing a resist pattern issuppressed.

In the second preferred embodiment of present invention, the averagedensity of the laminated structure of the electrode and the protectivemetal film may be about 1.5 times or greater than the density of thefirst insulating layer. With this arrangement, unwanted ripplesappearing in the resonance characteristic or the filter characteristicare shifted outside the pass band.

In the first or second preferred embodiment of the present invention,the first and second insulating layers may include SiO₂. Thus, a SAWapparatus having an improved TCF is provided.

In the first or second preferred embodiment of the present invention,the reflection of a SAW may be preferably utilized. The structure of aSAW apparatus utilizing the reflection of a SAW is not particularlyrestricted. and An end-surface-reflection-type SAW apparatus utilizingthe reflection of two opposing side surfaces of a piezoelectricsubstrate or a SAW apparatus provided with reflectors disposed tosandwich an electrode therebetween in the SAW propagating direction maybe used.

The SAW apparatus of the first or second preferred embodiment of thepresent invention can be used in various types of SAW resonators and SAWfilters. The SAW resonator may be a one-port resonator or a two-portresonator, and the SAW filter may be a two-port resonator filter, aladder filter, or a lattice filter.

In the first or second preferred embodiment of the present invention,the electrode may be an IDT electrode. The IDT electrode may be aunidirectional electrode in which the insertion loss can be reduced.Alternatively, the electrode may be a reflector.

In the first or second preferred embodiment of the present invention,the piezoelectric substrate may be a LiTaO₃ substrate having Eulerangles of about (0±3°, 104°-140°, 0±3°), the first and second insulatinglayers may include a SiO₂ film, the normalized thickness Hs/λ may rangefrom about 0.03 to about 0.45 where Hs is a total thickness of the SiO₂film of the first and second insulating layers and λ is the wavelengthof a SAW, and the normalized thickness H/λ of the electrode may satisfythe following equation (1):0.005≦H/λ≦0.00025×ρ2−0.01056×ρ+0.16473  Equation (1)where H indicates the thickness of the electrode and ρ represents theaverage density of the electrode.

Au, Ag, Cu, W, Ta, Pt, Ni, or Mo may be used in forming the electrode.

In preferred embodiments of the present invention, the electrode may bemade of one of the above-described metals or an alloy primarilyincluding such a metal, or formed of a laminated film including aprimary metallic film made of one of the above-described metals or analloy including one of the above-described metals. According to the typeof metal, the normalized thickness H/λ of the electrode, the Eulerangles of the piezoelectric substrate, and the total normalizedthickness Hs/λ of the first and second SiO₂ insulating layers aredefined to be within specific ranges, thereby improving theelectromechanical coupling coefficient, the reflection coefficient, andthe TCF. The attenuation constant can also be reduced.

According to a third preferred embodiment of the present invention, amethod of manufacturing the SAW apparatus according to the firstpreferred embodiment of the present invention includes preparing apiezoelectric substrate, forming a first insulating layer on theentirety of one surface of the piezoelectric substrate, removing, byusing a resist pattern for forming an electrode pattern including atleast one electrode, the at least a portion of the first insulatinglayer in an area in which the electrode is to be formed, and maintaininga laminated structure of the first insulating layer and the resistpattern in an area other than the area in which the electrode is to beformed; forming the at least one electrode by forming an electrode filmincluding at least one of a metal having a density higher than Al, analloy including a metal having a density higher than Al in an area ofthe portion of the first insulating layer which was removed so that thethickness of the electrode film becomes substantially the same as thethickness of the first insulating layer, removing the resist pattern onthe first insulating layer, and forming a second insulating layer tocover the first insulating layer and the electrode.

With this configuration, because the second insulating layer is formedto cover the first insulating layer and the electrode, there issubstantially no difference in the height of the top surface of thesecond insulating layer, thereby reducing the insertion loss.Additionally, because the electrode is made of a metal or an alloyhaving a density higher than Al, the reflection coefficient of theelectrode can be improved, thereby suppressing characteristicdeterioration caused by unwanted ripples.

In the manufacturing method of the third preferred embodiment of thepresent invention, the density of the metal or the alloy forming theelectrode may be about 1.5 times or greater than that of the firstinsulating layer. With this arrangement, unwanted ripples appearing inthe resonance characteristic or the filter characteristic are shiftedoutside the pass band.

According to a fourth preferred embodiment of the present invention, amethod of manufacturing the SAW apparatus of the second preferredembodiment of the present invention includes preparing a piezoelectricsubstrate, forming a first insulating layer on the entirety of onesurface of the piezoelectric substrate, removing a portion of the firstinsulating layer by using a resist pattern, maintaining a laminatedstructure of the first insulating layer and the resist pattern, formingat least one electrode by forming a metal or an alloy in an area of theportion of the first insulating layer which was removed, forming aprotective metal film made of a metal or an alloy having highererosion-resistant characteristics than the metal or the alloy of the atleast one electrode on the entire surface of the at least one electrodeso that the height of the protective metal film becomes substantiallythe same as the height of the first insulating layer, removing theresist pattern on the first insulating layer, and forming a secondinsulating layer to cover the protective metal film formed on theelectrode and the first insulating layer.

With this configuration, in the step of removing the resist pattern,because the side surfaces of the electrode are covered with the firstinsulating layer and the top surface is covered with the protectivemetal film, the erosion of the electrode can be suppressed.

In the fourth preferred embodiment of the present invention, the metalor the alloy forming the electrode and the metal or the alloy formingthe protective metal film may be selected so that the average density ofthe laminated structure of the electrode and the protective metal filmis about 1.5 times or greater than the density of the first insulatinglayer. With this arrangement, unwanted ripples appearing in theresonance characteristic or the filter characteristic are shiftedoutside the pass band.

According to a fifth preferred embodiment of the present invention, amethod of manufacturing a SAW apparatus includes preparing apiezoelectric substrate, forming an electrode on the piezoelectricsubstrate, forming an insulating layer to cover the electrode, andplanarizing a difference of the height of the insulating layer.Accordingly, a characteristic deterioration caused by the differences inthe height of the top surface of the insulating layer can be suppressed.

In the fifth preferred embodiment of the present invention, theplanarizing step may preferably be performed by an etch back process, areverse sputtering process, or a polishing process.

Other features, elements, characteristics and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments thereof with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1G are partially cut sectional views schematicallyillustrating a manufacturing method for a SAW apparatus according to afirst preferred embodiment of the present invention;

FIG. 2 illustrates the relationship between the reflection coefficientand the electrode thickness when the surface of a SiO₂ film isplanarized and when the surface of the SiO₂ film is not planarized of aone-port SAW resonator in which IDT electrodes made of aluminum (Al),Gold (Au), or platinum (Pt) having various thickness values and the SiO₂film having a normalized thickness of about 0.2 are formed on a LiTaO₃substrate having Euler angles (0°, 126°, 0°);

FIG. 3 illustrates the relationship between the reflection coefficientand the electrode thickness when the surface of a SiO₂ film isplanarized and when the surface of the SiO₂ film is not planarized of aone-port SAW resonator in which IDT electrodes made of Al, copper (Cu),or silver (Ag) having various thickness values and the SiO₂ film havinga normalized thickness of about 0.2 are formed on a LiTaO₃ substratehaving Euler angles (0°, 126°, 0°);

FIG. 4 illustrates the impedance and the phase with respect to thefrequency when the SiO₂ film of a SAW resonator formed by amanufacturing method of a first comparative example was changed;

FIG. 5 illustrates the relationship between the Figure of Merit (MF) ofthe resonator and the thickness of the SiO₂ film of the SAW resonator ofthe first comparative example;

FIG. 6 is a schematic plan view illustrating a one-port SAW resonatorobtained by the manufacturing method shown in FIGS. 1A through 1G;

FIG. 7 illustrates the impedance and the phase with respect to thefrequency when the SiO₂ film of the SAW resonator obtained by themanufacturing method of the first preferred embodiment of the presentinvention was changed;

FIG. 8 illustrates the relationship of γ of the SAW resonator obtainedby the manufacturing method of the first preferred embodiment of thepresent invention and that of the first comparative example to thethickness of the SiO₂ film;

FIG. 9 illustrates the relationship of the MF of the SAW resonatorobtained by the manufacturing method of the first preferred embodimentof the present invention and that of the first comparative example tothe thickness of the SiO₂ film;

FIG. 10 illustrates the relationship between the temperature coefficientof frequency (TCF) and the thickness of the SiO₂ film of the SAWresonator obtained by the manufacturing method of the first preferredembodiment of the present invention and that of the first comparativeexample;

FIG. 11 illustrates the impedance and the phase with respect to thefrequency of a SAW resonator with a SiO₂ film and a SAW resonatorwithout a SiO₂ SiO₂ film manufactured by a second comparative example;

FIGS. 12A through 12E illustrate the impedance with respect to thefrequency when the ratio of the average density of the IDT electrode andthe protective metal film to the density of the first insulating layerwas changed;

FIG. 13 illustrates a change in the electromechanical couplingcoefficient when IDT electrodes made of various metals having variousthickness values were formed on a LiTaO₃ substrate having Euler angles(0°, 126°, 0°);

FIG. 14 illustrates the relationship of the range of the electrodethickness that exhibits greater electromechanical coupling coefficientsthan Al to the density of the corresponding electrode when IDTs made ofvarious metals were formed on a LiTaO₃ substrate;

FIG. 15 is a plan view illustrating a SAW apparatus according to asecond preferred embodiment of the present invention;

FIG. 16 illustrates the relationship between the electromechanicalcoupling coefficient and the normalized thickness of IDTs when the IDTsmade of Au, Ta, Ag, Cr, W, Cu, Zn, Mo, Ni, and Al were formed on a36°-rotated Y-plate X-propagating LiTaO₃ substrate having Euler angles(0°, 126°, 0°);

FIG. 17 illustrates the relationship between the reflection coefficientof a single electrode finger of IDTs made of various electrode materialson a 36°-rotated Y-plate X-propagating LiTaO₃ substrate having Eulerangles (0°, 126°, 0°) and the thickness of the IDTs;

FIG. 18 illustrates the relationship between the attenuation constant αand the normalized thickness of IDTs when the IDTs made of Au, Ta, Ag,Cr, W, Cu, Zn, Mo, Ni, and Al were formed on a 36°-rotated Y-plateX-propagating LiTaO₃ substrate having Euler angles (0°, 126°, 0°);

FIG. 19 illustrates a change in the TCF with respect to the normalizedthickness of a SiO₂ film when an Au IDT having a normalized thickness of0.02 was formed on a 36°-rotated Y-plate X-propagating LiTaO₃ substratehaving Euler angles (0°, 126°, 0°);

FIG. 20 illustrates a change in the attenuation constant α with respectto the normalized thickness of a SiO₂ film when Au IDTs having variousthickness values were formed on a 36°-rotated Y-plate X-propagatingLiTaO₃ substrate having Euler angles (0°, 126°, 0°);

FIG. 21 illustrates a change in the attenuation constant α with respectto the normalized thickness of a SiO₂ film when Au IDTs having variousthickness values were formed on a 38°-rotated Y-plate X-propagatingLiTaO₃ substrate having Euler angles (0°, 128°, 0°);

FIG. 22 illustrates the attenuation-vs.-frequency of the SAW apparatusof the first preferred embodiment of the present invention before andafter a SiO₂ film was formed;

FIG. 23 illustrates a change in the acoustic velocity of a leaky SAWwith respect to the thickness of an Au IDT when the Au IDT and SiO₂films having various thickness values were formed on a 36°-rotatedY-plate X-propagating LiTaO₃ substrate having Euler angles (0°, 126°,0°);

FIG. 24 illustrates a change in the acoustic velocity of a leaky SAWwith respect to the thickness of a SiO₂ film when Au IDTs having variousthickness values and the SiO₂ film were formed on a 36°-rotated Y-plateX-propagating LiTaO₃ substrate having Euler angles (0°, 126°, 0°);

FIG. 25 illustrates a change in the electromechanical couplingcoefficient with respect to Θ of Euler angles (0°, Θ, 0°) when thenormalized thickness of an Au IDT and the normalized thickness of a SiO₂film were changed;

FIG. 26 illustrates a change in the Q factor with respect to Θ of Eulerangles (0°, Θ, 0°) when the normalized thickness of a SiO₂ film waschanged;

FIGS. 27A through 27C are schematic sectional views illustrating a SAWapparatus of a modified example of preferred embodiments of the presentinvention provided with a contact layer;

FIG. 28 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.1 and when Au electrodes having various thicknessvalues were formed;

FIG. 29 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.15 and when Au electrodes having various thicknessvalues were formed;

FIG. 30 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.12 and when Au electrodes having various thicknessvalues are formed;

FIG. 31 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.25 and when Au electrodes having various thicknessvalues were formed;

FIG. 32 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.3 and when Au electrodes having various thicknessvalues were formed;

FIG. 33 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.35 and when Au electrodes having various thicknessvalues were formed;

FIG. 34 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.40 and when Au electrodes having various thicknessvalues were formed;

FIG. 35 illustrates a change in the attenuation constant α with respectto Θ of Euler angles (0°, Θ, 0°) when the normalized thickness of a SiO₂film was about 0.45 and when Au electrodes having various thicknessvalues were formed;

FIG. 36 illustrates the relationship between the electromechanicalcoupling coefficient and the normalized thickness of an Ag electrodeformed on a LiTaO₃ substrate having Euler angles (0°, 126°, 0°)according to a third preferred embodiment of the present invention;

FIG. 37 illustrates the relationship between the TCF and the normalizedthickness of SiO₂ films formed on three LiTaO₃ substrate having Eulerangles (0°, 113°, 0°), (0°, 126°, 0°), and (0°, 129°, 0°);

FIG. 38 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and SiO₂ filmshaving a normalized thickness of 0 to about 0.5 were formed on a LiTaO₃substrate having Euler angles (0°, 120°, 0°);

FIG. 39 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and SiO₂ filmshaving a normalized thickness of 0 to about 0.5 were formed on a LiTaO₃substrate having Euler angles (0°, 140°, 0°);

FIG. 40 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ SiO₂film having a normalized thickness of about 0.1 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 41 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.15 were formed on a LiTaO₃LiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 42 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.2 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 43 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.25 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 44 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.3 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 45 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.35 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 46 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.4 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 47 illustrates a change in the attenuation constant α when Ag filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.45 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 48 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and SiO₂ filmshaving a normalized thickness of 0 to about 0.5 were formed on a LiTaO₃substrate having Euler angles (0°, 120°, 0°) according to a fourthpreferred embodiment of the present invention;

FIG. 49 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and SiO₂ filmshaving a normalized thickness of 0 to about 0.5 were formed on a LiTaO₃substrate having Euler angles (0°, 135°, 0°);

FIG. 50 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.1 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 51 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.15 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 52 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.2 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 53 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.25 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 54 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.3 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 55 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO2 filmhaving a normalized thickness of about 0.35 were formed on a LiTaO₃LiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 56 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.4 were formed on a LiTaO₃substrate having Euler angles (0°, Θ, 0°);

FIG. 57 illustrates a change in the attenuation constant α when Cu filmshaving a normalized thickness of about 0.1 or smaller and a SiO₂ filmhaving a normalized thickness of about 0.45 were formed on a LiTaO3substrate having Euler angles (0°, Θ, 0°);

FIG. 58 illustrates the relationship between the reflection coefficientper electrode finger of an Al electrode and that of a Cu electrode andthe normalized thickness of the corresponding electrode when thenormalized thickness of a SiO₂ film was about 0.02;

FIG. 59 illustrates the relationship between Θmin that reduces theattenuation constant α to 0 or minimizes the attenuation constant α andthe normalized thickness of a SiO₂ film when the normalized thickness ofa Cu film was changed;

FIG. 60 illustrates a change in the attenuation constant α when SiO₂films having various thickness values and tungsten IDTs having variousthickness values were formed on a LiTaO₃ substrate (0°, 120°, 0°)according to a fifth preferred embodiment of the present invention;

FIG. 61 illustrates a change in the attenuation constant α when SiO₂films having various thickness values and tungsten IDTs having variousthickness values were formed on a LiTaO₃ substrate (0°, 140°, 0°);

FIG. 62 illustrates the relationship of the attenuation constant α to Θand the thickness of tungsten electrodes when the tungsten electrodeshaving various thickness values and a SiO₂ film having a normalizedthickness of about 0.1 were formed on a LiTaO₃ substrate having Eulerangles (0°, Θ, 0°);

FIG. 63 illustrates the relationship of the attenuation constant α to Θand the thickness of tungsten electrodes when the tungsten electrodeshaving various thickness values and a SiO₂ film having a normalizedthickness of about 0.2 were formed on a LiTaO₃ substrate having Eulerangles (0°, Θ, 0°);

FIG. 64 illustrates the relationship of the attenuation constant α to Θand the thickness of tungsten electrodes when the tungsten electrodeshaving various thickness values and a SiO₂ film having a normalizedthickness of about 0.3 were formed on a LiTaO₃ substrate having Eulerangles (0°, Θ, 0°);

FIG. 65 illustrates the relationship of the attenuation constant α to Θand the thickness of tungsten electrodes when the tungsten electrodeshaving various thickness values and a SiO₂ film having a normalizedthickness of about 0.4 were formed on a LiTaO₃ substrate having Eulerangles (0°, Θ, 0°);

FIG. 66 illustrates the relationship between the acoustic velocity andthe normalized thickness of SiO2 films when tungsten films havingvarious thickness values and the SiO₂ films are formed on a LiTaO₃substrate having Euler angles (0°, 126°, 0°);

FIG. 67 illustrates the relationship between the acoustic velocity andthe normalized thickness of tungsten films when the tungsten films andSiO₂ films having various thickness values were formed on a LiTaO₃substrate having Euler angles (0°, 126°, 0°);

FIG. 68 illustrates a change in the attenuation constant α when tantalumIDTs having various thickness values and SiO₂ films having variousthickness values were formed on a LiTaO₃ substrate having Euler angles(0°, 120°, 0°) according to a sixth preferred embodiment of the presentinvention;

FIG. 69 illustrates a change in the attenuation constant α when tantalumIDTs having various thickness values and SiO₂ films having variousthickness values were formed on a LiTaO₃ substrate having Euler angles(0°, 140°, 0°);

FIG. 70 illustrates the relationship between the attenuation constant αand Θ when tantalum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.1 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 71 illustrates the relationship between the attenuation constant αand Θ when tantalum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.2 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 72 illustrates the relationship between the attenuation constant αand Θ when tantalum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.3 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 73 illustrates the relationship between the attenuation constant αand Θ when tantalum electrode films having various thickness values anda SiO2 film having a normalized thickness of about 0.4 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 74 illustrates the relationship between the acoustic velocity andthe normalized thickness of SiO₂ films when tantalum IDTs having variousthickness values and the SiO₂ films were formed on a LiTaO₃ substratehaving Euler angles (0°, 126°, 0°);

FIG. 75 illustrates the relationship between the acoustic velocity andthe normalized thickness of tantalum IDTs when the tantalum IDTs andSiO₂ films having various thickness values were formed on a LiTaO3substrate having Euler angles (0°, 126°, 0°);

FIG. 76 illustrates a change in the attenuation constant α when platinumIDTs having various thickness values and SiO₂ films having variousthickness values were formed on a LiTaO₃ substrate having Euler angles(0°, 125°, 0°) according to a seventh preferred embodiment of thepresent invention;

FIG. 77 illustrates a change in the attenuation constant α when platinumIDTs having various thickness values and SiO2 films having variousthickness values are formed on a LiTaO₃ substrate having Euler angles(0°, 140°, 0°);

FIG. 78 illustrates the relationship between the attenuation constant αand Θ when platinum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.1 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 79 illustrates the relationship between the attenuation constant αand Θ when platinum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.15 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 80 illustrates the relationship between the attenuation constant αand Θ when platinum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.2 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 81 illustrates the relationship between the attenuation constant αand Θ when platinum electrode films having various thickness values anda SiO₂ film having a normalized thickness of about 0.25 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 82 illustrates the relationship between the attenuation constant αand Θ when platinum electrode films having various thickness values anda SiO2 film having a normalized thickness of about 0.3 were formed on aLiTaO3 substrate having Euler angles (0°, Θ, 0°);

FIG. 83 illustrates the relationship between the attenuation constant αand Θ when platinum electrode films having various thickness values anda SiO2 film having a normalized thickness of about 0.4 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 84 illustrates the relationship between the acoustic velocity andthe normalized thickness of SiO₂ films when platinum IDTs having variousthickness values and the SiO₂ films were formed on a LiTaO₃ LiTaO₃substrate having Euler angles (0°, 126°, 0°);

FIG. 85 illustrates the relationship between the acoustic velocity andthe normalized thickness of platinum IDTs when the platinum IDTs andSiO₂ films having various thickness values were formed on a LiTaO₃substrate having Euler angles (0°, 126°, 0);

FIG. 86 illustrates a change in the attenuation constant α when nickelIDTs having various thickness values and SiO₂ films having variousthickness values were formed on a LiTaO₃ substrate having Euler angles(0°, 120°, 0°) according to an eighth preferred embodiment of thepresent invention;

FIG. 87 illustrates a change in the attenuation constant α when nickelIDTs having various thickness values and SiO₂ films having variousthickness values were formed on a LiTaO₃ substrate having Euler angles(0°, 140°, 0°);

FIG. 88 illustrates a change in the attenuation constant α whenmolybdenum IDTs having various thickness values and SiO₂ films havingvarious thickness values were formed on a LiTaO₃ substrate having Eulerangles (0°, 120°, 0°);

FIG. 89 illustrates a change in the attenuation constant α whenmolybdenum IDTs having various thickness values and SiO₂ films havingvarious thickness values were formed on a LiTaO₃ substrate having Eulerangles (0°, 140°, 0°);

FIG. 90 illustrates the relationship between the attenuation constant αand Θ when nickel electrode films having various thickness values and aSiO₂ film having a normalized thickness of about 0.1 were formed on aLiTaO3 substrate having Euler angles (0°, Θ, 0°);

FIG. 91 illustrates the relationship between the attenuation constant αand Θ when nickel electrode films having various thickness values and aSiO₂ film having a normalized thickness of about 0.2 were formed on aLiTaO3 substrate having Euler angles (0°, Θ, 0°);

FIG. 92 illustrates the relationship between the attenuation constant αand Θ when nickel electrode films having various thickness values and aSiO₂ film having a normalized thickness of about 0.3 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 93 illustrates the relationship between the attenuation constant αand Θ when nickel electrode films having various thickness values and aSiO₂ film having a normalized thickness of about 0.4 were formed on aLiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 94 illustrates the relationship between the attenuation constant αand Θ when molybdenum electrode films having various thickness valuesand a SiO₂ film having a normalized thickness of about 0.1 were formedon a LiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 95 illustrates the relationship between the attenuation constant αand Θ when molybdenum electrode films having various thickness valuesand a SiO₂ film having a normalized thickness of about 0.2 were formedon a LiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 96 illustrates the relationship between the attenuation constant αand Θ when molybdenum electrode films having various thickness valuesand a SiO₂ film having a normalized thickness of about 0.3 are formed ona LiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 97 illustrates the relationship between the attenuation constant αand Θ when molybdenum electrode films having various thickness valuesand a SiO2 film having a normalized thickness of about 0.4 were formedon a LiTaO₃ substrate having Euler angles (0°, Θ, 0°);

FIG. 98 illustrates the relationship between the acoustic velocity andthe normalized thickness of nickel IDTs when the nickel IDTs and SiO₂films having various thickness values were formed on a LiTaO₃ substratehaving Euler angles (0°, 126°, 0°);

FIG. 99 illustrates the relationship between the acoustic velocity andthe normalized thickness of SiO₂ films when nickel IDTs having variousthickness values and the SiO₂ films were formed on a LiTaO₃ substratehaving Euler angles (0°, 126°, 0°);

FIG. 100 illustrates the relationship between the acoustic velocity andthe normalized thickness of molybdenum IDTs when the molybdenum IDTs andSiO₂ films having various thickness values were formed on a LiTaO₃substrate having Euler angles (0°, 126°, 0°);

FIG. 101 illustrates the relationship between the acoustic velocity andthe normalized thickness of SiO₂ films when molybdenum IDTs havingvarious thickness values and the SiO₂ films were formed on a LiTaO₃LiTaO₃ substrate having Euler angles (0°, 126°, 0°);

FIGS. 102A through 102C are schematic sectional views illustrating anetch back process for planarizing the surface of an insulating layer;

FIGS. 103A through 103D are schematic sectional views illustrating areverse sputtering process for planarizing the surface of an insulatinglayer;

FIGS. 104A and 104B are schematic sectional views illustrating anotherprocess for planarizing the surface of an insulating layer;

FIGS. 105A through 105C are schematic sectional views illustrating stillanother process for planarizing the surface of an insulating layer;

FIGS. 106A and 106B are schematic plan views illustrating a one-port SAWresonator and a two-port SAW resonator, respectively, to which thepresent invention is applied;

FIG. 107 is a schematic plan view illustrating a ladder filter to whichthe present invention is applied;

FIG. 108 is a schematic plan view illustrating a lattice filter to whichthe present invention is applied;

FIGS. 109A through 109D are schematic sectional views illustrating oneexample of a known manufacturing method for a SAW apparatus; and

FIG. 110 illustrates a schematic sectional view illustrating anotherexample of a known manufacturing method for a SAW apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is described in detail below with reference to theaccompanying drawings through illustration of preferred embodiments ofthe present invention.

A manufacturing method for a SAW apparatus according to a firstpreferred embodiment of the present invention is described below withreference to FIGS. 1A through 1G, and 6.

As shown in FIG. 1A, a LiTaO₃ substrate 1 is first prepared as apiezoelectric substrate. In the first preferred embodiment of thepresent invention, a 36°-rotated Y-plate X-propagating LiTaO₃ substratehaving Euler angles (0°, 126°, 0°) is preferably used. As thepiezoelectric substrate, a LiTaO3 substrate having different crystalorientations may be used, or a substrate made of another piezoelectricsingle crystal may be used. Alternatively, a piezoelectric substrateformed by laminating piezoelectric thin films on an insulating substratemay be used. Θ of the Euler angles (φ, Θ, ψ) can be expressed by Θ=cutangle+90°.

As shown in FIG. 1A, a first insulating layer 2 is formed on the entiresurface of the LiTaO₃ substrate 1. In this preferred embodiment, thefirst insulating layer 2 is preferably formed of a SiO₂ film. The firstinsulating layer 2 is formed according to a suitable technique such asprinting, deposition, or sputtering. The thickness of the firstinsulating layer 2 is preferably about equal to that of an IDTelectrode, which is formed in a later step.

Then, as shown in FIG. 1B, a resist pattern 3 is formed according to aphotolithographic technique in an area other than an area in which anIDT electrode is to be formed.

Subsequently, as indicated by the arrows in FIG. 1C, as a result ofreactive ion etching (RIE) by applying ion beams, the first insulatinglayer 2 is removed, except for the portion disposed under the resistpattern 3.

When the SiO₂ film (first insulating layer 2) is etched by RIE using afluorinated gas, a residue may be generated after polymerization. Inthis case, a buffered hydrofluoric acid (BHF) may be applied afterperforming RIE.

Thereafter, a Cu film and a Ti film are formed such that the thicknessthereof is about equal to that of the first insulating layer 2. Morespecifically, as shown in FIG. 1D, a Cu film 4, which serves as an IDTelectrode 4A, is formed on the area without the first insulating layer2, and the Cu film 4 is also formed on the resist pattern 3. Then, asshown in FIG. 1E a Ti film 5 is formed as a protective metal film on thetop surface of an IDT electrode 4A and on the Cu film 4 formed on theresist pattern 3. Accordingly, the side surfaces of the IDT electrode 4Aare covered with the first insulating layer 2, and the top surfacethereof is covered with the Ti film 5. As discussed above, the IDTelectrode 4A and the protective metal film (Ti film 5) are formed suchthat the total thickness of the IDT electrode 4A and the Ti film 5 areabout equal to the thickness of the first insulating layer 2.

Subsequently, the resist pattern 3 is removed by using a resiststripper. Then, as shown in FIG. 1F, the IDT electrode 4A is disposed inan area other than the area in which the first insulating layer 2 isformed, and the top surface of the IDT electrode 4A is covered with theTi film 5.

Thereafter, as shown in FIG. 1G, a SiO₂ film, which serves as a secondinsulating layer 6, is formed on the entire surface of the SAWapparatus.

A one-port SAW resonator 11 shown in FIG. 6 is fabricated.

In FIGS. 1A through 1G, only the portion in which the IDT electrode 4Ais formed is shown. However, as shown in FIG. 6, the SAW resonator 11 isalso provided with reflectors 12 and 13 in a SAW propagating directionsuch that they sandwich the IDT electrode 4A therebetween. Thereflectors 12 and 13 are also formed in the same steps as those of theIDT electrodes 4A.

In the above-described first preferred embodiment of the presentinvention, because the one-port SAW resonator 11 is formed, only one IDTelectrode 4A is formed on the LiTaO₃ substrate 1. However, a pluralityof IDT electrodes may be formed according to the intended purpose of theSAW apparatus. Reflectors and an IDT electrode may be simultaneouslyformed. Alternatively, reflectors do not have to be formed.

For comparison, a one-port SAW resonator was formed as a firstcomparative example according to the known manufacturing method shown inFIGS. 109A through 109D for a SAW apparatus with a SiO₂ film. As in thefirst preferred embodiment of the present invention, a 36°-rotatedY-plate X-propagating LiTaO₃ LiTaO₃ substrate having Euler angles (0°,126°, 0°) was used. An IDT electrode was formed by using Cu. Accordingto the manufacturing method shown in FIGS. 109A through 109D, becausethe SiO₂ film 54 is formed after the IDT electrode 53A, the height ofthe SiO₂ film 54 is not uniform. FIG. 4 shows the impedance and thephase of the first comparative example when the normalized thickness h/λ(h represents the thickness of the IDT electrode 53A and λ designatesthe SAW wavelength) of the IDT electrode 53A is 0.042 and when thenormalized thickness Hs/λ (Hs represents the thickness of the SiO₂ film54) of the SiO₂ film 54 is 0.11, 0.22, and 0.33. FIG. 4 shows that theratio of the impedance of the antiresonance point to the impedance ofthe resonance point, i.e., the impedance ratio, becomes smaller as thenormalized thickness Hs/λ of the SiO₂ film 54 is increased.

FIG. 5 illustrates the relationship between the normalized thicknessHs/λ of the SiO₂ film 54 of the SAW resonator manufactured by the knownmethod in the first comparative example and the Figure of Merit (MF) ofthe SAW resonator. FIG. 5 reveals that MF is decreased as the normalizedthickness Hs/λ of the SiO₂ film 54 becomes larger.

That is, in the first comparative example, the characteristics of theresonator is considerably decreased as the thickness of the SiO₂ film 54is increased, even if the IDT electrode 53A is made of Cu. This isprobably due to the difference of the surface height of the SiO₂ film54.

The characteristics of the SAW resonator 11 manufactured according tothe first preferred embodiment of the present invention are shown inFIGS. 7 through 9.

FIG. 7 illustrates a change in the impedance and a change in the phaseof the SAW resonator 11 manufactured according to the method of theabove-described first preferred embodiment of the present invention whenthe thickness of the SiO₂ film, the second insulating layer 6, ischanged. The two-dot-chain lines in FIGS. 8 and 9 indicate a change in γand a change in MF of the SAW resonator 11, respectively, when thenormalized thickness Hs/λ of the SiO₂ film is varied. For comparison,the corresponding characteristics of the known resonator manufactured inthe first comparative example are also indicated by the solid lines inFIGS. 8 and 9.

By comparing the characteristics of FIG. 7 with those of FIG. 4, it isseen that a decrease in the impedance is small even though thenormalized thickness Hs/λ of the SiO₂ film is increased.

FIGS. 8 and 9 also show that a characteristic deterioration of thispreferred embodiment can be suppressed even though the normalizedthickness Hs/λ of the SiO₂ film is increased.

That is, according to the manufacturing method of the first preferredembodiment of the present invention, a decrease in the impedance ratiois small and a characteristic deterioration can be suppressed eventhough the thickness of the SiO₂ film is increased.

FIG. 10 illustrates the relationship between the temperature coefficientof frequency (TCF) of the SAW resonator of the first preferredembodiment of the present invention and that of the first comparativeexample and the thickness of the SiO₂ film. In FIG. 10, the solid lineindicates the first comparative example, and the two-dot-chain lineindicates the first preferred embodiment of the present invention.

FIG. 10 indicates that the TCF can be ideally improved by increasing thethickness of the SiO₂ film according to the manufacturing method of thefirst preferred embodiment of the present invention.

Thus, according to the manufacturing method of the first preferredembodiment of the present invention, it is possible to provide a SAWresonator in which the characteristic deterioration can be suppressedand the TCF can be effectively improved.

Additionally, according to the manufacturing method of the firstpreferred embodiment of the present invention, because the IDT electrode4A is made of Cu, which has a density higher than Al, it has asufficient reflection coefficient, thereby suppressing the generation ofundesirable ripples in the resonance characteristic. This is describedin detail below.

For comparison, a SAW resonator was formed as a second comparativeexample in a manner similar to the first preferred embodiment of thepresent invention, except that Al was used for the IDT electrode insteadof Cu and the normalized thickness Hs/λ of the SiO₂ film, the firstinsulating layer, was about 0.08. The impedance and the phase of the SAWresonator of the second comparative example are indicated by the solidlines of FIG. 11.

The impedance and the phase of the SAW resonator formed in a mannersimilar to the second comparative example, except that a SiO₂ film wasnot formed, are indicated by the broken lines in FIG. 11.

The solid lines of FIG. 11 indicate that large ripples (indicated by thearrows A) are generated between the resonance point and theantiresonance point when the IDT electrode was formed of Al and the SiO₂film was formed in the second comparative example. In contrast, thebroken lines of FIG. 11 indicate that such ripples are not generated inthe SAW resonator without the SiO₂ film.

Accordingly, even though the SiO₂ film was formed to improve the TCF,the above-described ripples A are generated if the IDT electrode isformed by using Al, resulting in a characteristic deterioration. Afterfurther studying this point, the present inventors discovered that thereflection coefficient of the IDT electrode can be increased by using ametal having a density higher than Al for the IDT electrode so as tosuppress the above-described ripples A.

Then, a SAW resonator was formed in a manner similar to theabove-described first preferred embodiment of the present invention,except that the density of the metal for the IDT electrode was varied.The impedances of the SAW resonators using metals with differentdensities are shown in FIGS. 12A through 12E. FIGS. 12A through 12Eillustrate the impedances when the ratio ρ1/ρ2 of the average density ρ1of the laminated structure including the IDT electrode and theprotective metal film to the density ρ2 of the first insulating layer isabout 2.5, 2.0, 1.5, 1.2, and 1.0, respectively.

FIGS. 12A through 12C show that the ripples A are shifted to the rangeoutside the pass band, and more particularly, FIG. 12A shows that theripples A are considerably suppressed.

Accordingly, as is seen from FIGS. 12A through 12E, the ripples A can beshifted to the range outside the band pass between the resonantfrequency and the antiresonant frequency when the density ratio of thelaminated structure including the IDT electrode and the protective metalfilm to the first insulating layer is about 1.5 or greater, therebyhaving improved characteristics. When the density ratio is about 2.5 orgreater, the ripples can be considerably suppressed.

In the examples of FIGS. 12A through 12E, because the Ti film islaminated on the IDT electrode 4A, the average density was calculated.However, in the preferred embodiments of the present invention, theprovision of the protective metal film on an IDT is not essential. Inthis case, the thickness of the IDT electrode is preferably about equalto that of the first insulating layer, and the ratio of the density ofthe IDT electrode to that of the first insulating layer is preferablyabout 1.5 or greater, and more preferably, about 2.5 or greater. Then,advantages similar to those obtained by the above-described example canbe achieved.

Accordingly, in a SAW resonator in which a SiO₂ film is formed to coveran IDT electrode, the reflection coefficient of the IDT electrode can beincreased if the density of the IDT electrode, or the average density ofa laminated structure including the IDT electrode and a protective metalfilm, is preferably greater than the density of a first insulating layerdisposed along the side surfaces of the IDT electrode, therebysuppressing the generation of ripples between the resonance point andthe antiresonance point.

A metal or an alloy having a higher density than Al includes, not onlyCu, but also Ag or Au or an alloy essentially consisting of Ag or Au.

As in the first preferred embodiment of the invention, a protectivemetal film is preferably disposed on the IDT electrode. Then, accordingto the manufacturing method shown in FIGS. 1A through 1G, when theresist pattern 3 is removed, the erosion of the IDT electrode 4A can beprevented because the side surfaces of the IDT electrode 4A are coveredwith the first insulating layer 2 and the top surface thereof is coveredwith the protective metal film 5. It is thus possible to provide a SAWresonator having superior characteristics.

The first and second insulating layers may be formed by an insulatingmaterial other than SiO₂, such as SiOxNy, which contributes to animprovement in the temperature characteristics. The first and secondinsulating layers may be made of the same insulating materials, as thefirst preferred embodiment of the present invention, or they may beformed of different insulating materials.

FIG. 13 illustrates the relationship between the electromechanicalcoupling coefficient and the normalized thickness H/λ of IDT electrodesmade of various metals and having various thickness values on a LiTaO₃substrate having Euler angles (0°, 126°, 0°).

The types of metals having larger electromechanical coefficients than Alwere extracted from FIG. 13, and the normalized thickness values of suchmetals are shown in FIG. 14. That is, FIG. 14 illustrates the electrodethickness range that exhibits greater electromechanical coefficientsthan Al.

In FIG. 14, the upper limit of the thickness range of the IDT electrodesindicates the threshold for having a greater electromechanical couplingcoefficient than Al, and the lower limit of the thickness rangerepresents the thickness of the IDT electrode that can be manufactured.By approximating the upper limit to a quadratic expression when theelectrode thickness range having greater electromechanical couplingcoefficients is y and the density is x, the equationy=0.00025x2−0.01056x+0.16473 can be found.

Accordingly, as is seen from subsequent preferred embodiments in whichSAW resonators are formed by specifying electrode materials, it is nowassumed that an IDT electrode is formed on a 14°-50°-rotated Y-plateX-propagating LiTaO₃ piezoelectric substrate having Euler angles (0°,104°-140°, 0°), and the normalized thickness Hs/λ of a SiO₂ film rangesfrom about 0.03 to about 0.45. In this case, the electromechanicalcoupling coefficient can be increased, as shown in FIG. 14, when thenormalized thickness H/λ of the IDT electrode satisfies the followingexpression (1):0.005≦H/λ≦0.00025×ρ2−0.01056×ρ+0.16473  (1)wherein ρ represents the average density of the IDT electrode.

In preferred embodiments of the present invention, a metal having ahigher density than Al is preferably used for the IDT electrode. In thiscase, the IDT electrode may be made of a metal having a higher densitythan Al or an alloy primarily including a metal having a higher densitythan Al. Alternatively, the IDT electrode may be formed of a laminatedstructure including a primary metallic film made of a metal having ahigher density than Al or an alloy primarily including a metal having ahigher density than Al and a secondary metallic film made of a metaldifferent from that of the primary metallic film. In this case, theaverage density of the laminated film preferably satisfies theexpression represented by ρ0×0.7≦ρ≦ρ0×1.3 where ρ indicates the averagedensity of the IDT electrode and ρ0 designates the density of theprimary metallic film.

In preferred embodiments of the present invention, as described above,the surface of the second insulating layer is planarized. However, theheight of the second insulating film may be different within a range ofabout 30% or smaller of the thickness of the IDT electrode. If thisheight difference exceeds about 30%, the advantage achieved by theplanarized level of the second insulating layer cannot be sufficientlyobtained.

The second insulating layer can be planarized by various techniques,such as by performing an etch back process, by utilizing an obliqueincidence effect by means of reverse sputtering, by polishing thesurface of the insulating layer, and by polishing the electrode. Theplanarization of the second insulating layer may be performed by acombination of two or more types of the above-described techniques.Details of such techniques are discussed below with reference to FIGS.102A through 105C.

FIGS. 102A through 102C are schematic sectional views illustrating aplanarization technique for the surface of the insulating layeraccording to an etch back process. As shown in FIG. 102A, an electrode42 is first formed on a piezoelectric substrate 41, and then, aninsulating layer 43 is formed of, for example, SiO₂. As shown in FIG.102B, a resist pattern 44 is formed on the insulating layer 43 by, forexample, spin coating. In this case, the surface of the resist pattern44 is flat. Thus, by etching the resist pattern 44 according to RIE,i.e., by an etch back process, the surface of the insulating layer 43can be planarized, as shown in FIG. 102C.

FIGS. 103A through 103D are schematic sectional views illustrating thereverse sputtering process. The electrode 42 is first formed on thepiezoelectric substrate 41, and then, the insulating layer 43 is formed.Then, argon ions, which are used for sputtering the substrate 41, areapplied onto the surface of the insulating layer 43 by sputtering. Whensputtering is performed by ion bombardment on the surface of thesubstrate, a greater sputtering effect is produced if ions are appliedonto an oblique surface rather than a flat surface. This is known as the“oblique incidence effect”. Due to this effect, the insulating layer 43is planarized as the sputtering proceeds, as shown in FIGS. 103B through103D.

FIGS. 104A and 104B are schematic sectional views illustrating aplanarization technique by polishing the insulating layer. As shown inFIG. 104A, after the electrode 42 and the insulating layer 43 are formedon the substrate 41, the insulating layer 43 is mechanically orchemically polished so as to be planarized.

FIGS. 105A through 105C are schematic sectional views illustrating aplanarization technique by polishing the electrode. As shown in FIG.105A, a first insulating layer 45 is formed on the substrate 41, and ametallic film 42A, which is made of an electrode material, is formed onthe entire surface by deposition. Then, as shown in FIG. 105B, bymechanically or chemically polishing the metallic film 42A, theelectrode 42 and the first insulating layer 45, which is disposed aroundthe electrode 42, are formed. Thus, the first insulating layer 45 andthe electrode 42 are planarized so that they are flush with each other.Thereafter, as shown in FIG. 105C, a second insulating layer 46 isformed. According to this technique, the surface of the insulating layeris planarized.

The present invention is applicable to various types of SAW apparatuses.Examples of such SAW apparatuses are shown in FIGS. 106A through 108.FIGS. 106A and 106B are schematic plan views illustrating a one-port SAWresonator 47 and a two-port SAW resonator 48, respectively. By using thesame electrode structure as that of the two-port SAW resonator 48 shownin FIG. 106B, a two-port SAW resonator filter may be formed.

FIGS. 107 and 108 are schematic plan views illustrating the electrodestructures of a ladder filter 49 a and a lattice filter 49 b,respectively. By forming the electrode structure of the ladder filter 49a and the lattice filter 49 b on the piezoelectric substrate, a ladderfilter and a lattice filter can be formed according to the presentinvention.

The present invention is not restricted to the SAW apparatuses havingthe electrode structures shown in FIGS. 106A, 106B, and 107, and may beused in various types of SAW apparatuses.

In preferred embodiments of the present invention, preferably, a SAWapparatus using a leaky SAW is manufactured. Japanese Unexamined PatentApplication Publication No. 6-164306 discloses a SAW apparatus having anelectrode made of a heavy metal, such as Au, and utilizing the Lovewave, which is free from the propagation attenuation. In this SAWapparatus, by using a heavy metal for the electrode, the acousticvelocity of a propagating SAW becomes lower than that of a transversalbulk wave in the substrate so as to eliminate leaky components. In thismanner, the Love wave is utilized as a non-leaky SAW.

In the Love wave, however, because the acoustic velocity inevitablybecomes low, and accordingly, the IDT pitch must be decreased. Thisincreases the difficulty in processing the SAW apparatus, therebydecreasing the processing precision. Additionally, the linewidth of theIDT becomes smaller, and the loss caused by the resistance is increased.

In preferred embodiments of the present invention, unlike theabove-described SAW apparatus utilizing the Love wave, even though theelectrode made of a metal heavier than Al is used, a leaky SAW having ahigh acoustic velocity can be effectively utilized, thereby achieving areduction in the propagation loss. It is thus possible to provide alow-insertion SAW apparatus.

Based on the above-described results, electrodes were formed by usingdifferent metals having a higher density than Al.

Metals having a higher density than Al used in preferred embodiments thepresent invention include, for example:

-   -   (1) a metal having a density of 15000 to 23000 kg/m3 and a        Young's modulus of 0.5×1011 to 1.0×1011N/m² or having a        transversal-wave acoustic velocity of 1000 to 2000 m/s, for        example, Au;    -   (2) a metal having a density of 5000 to 15000 kg/m3 and a        Young's modulus of 0.5×1011 to 1.0×1011N/m² or having a        transversal-wave acoustic velocity of 1000 to 2000 m/s, for        example, Ag;    -   (3) a metal having a density of 5000 to 15000 kg/m3 and a        Young's modulus of 1.0×1011 to 2.05×1011N/m² or having a        transversal-wave acoustic velocity of 2000 to 2800 m/s, for        example, Cu;    -   (4) a metal having a density of 15000 to 23000 kg/m3 and a        Young's modulus of 2.0×1011 to 4.5×1011N/m² or having a        transversal-wave acoustic velocity of 2800 to 3500 m/s, for        example, tungsten;    -   (5) a metal having a density of 15000 to 23000 kg/m3 and a        Young's modulus of 1.0×1011 to 2.0×1011N/m² or having a        transversal-wave acoustic velocity of 2000 to 2800 m/s, for        example, tantalum;    -   (6) a metal having a density of 15000 to 23000 kg/m3 and a        Young's modulus of 1.0×1011 to 2.0×1011N/m² or having a        transversal-wave acoustic velocity of 1000 to 2000 m/s, for        example, platinum; and    -   (7) a metal having a density of 5000 to 15000 kg/m3 and a        Young's modulus of 2.0×1011 to 4.5×1011N/m² or having a        transversal-wave acoustic velocity of 2800 to 3500 m/s, for        example, Ni and Mo.

FIG. 15 is a plan view illustrating a longitudinally coupled resonatorfilter as a SAW apparatus 21 according to a second preferred embodimentof the present invention. In the second preferred embodiment of thepresent invention, Au is preferably used for electrodes.

In the SAW apparatus 21, IDTs 23 a and 23 b and reflectors 24 a and 24 bare formed on the top surface of a LiTaO₃ substrate 22. A SiO₂ film 25is formed to cover the IDTs 23 a and 23 b and the reflectors 24 a and 24b. As the LiTaO₃ substrate 22, a 25°-58°-rotated Y-plate X-propagatingLiTaO₃ substrate having Euler angles (0°, 115°-148°, 0°) is preferablyused. If a Y-plate X-propagating LiTaO₃ substrate having a cut angleother than the above range is used, the attenuation constant isincreased, and the TCF is deteriorated.

The IDTs 23 a and 23 b and the reflectors 24 a and 24 b are made of ametal having a density higher than Al. At least one metal selected fromthe group including Au, Pt, W, Ta, Ag, Mo, Cu, Ni, Co, Cr, Fe, Mn, Zn,and Ti, or an alloy primarily including at least one metal of theabove-described group may be used as the metal having a density higherthan Al.

As described above, because the IDTs 23 a and 23 b and the reflectors 24a and 24 b are made of a metal having a density higher than Al, theelectromechanical coupling coefficient and the reflection coefficientare improved, as shown in FIGS. 16 and 17, respectively, even when thethickness of the IDTs 23 a and 23 b and that of the reflectors 24 a and24 b are formed to be smaller compared to the IDTs and the reflectorsmade of Al.

The thickness of the electrodes can be decreased, as stated above. Thethickness of the SiO₂ film 25 is preferably determined so that thethickness Hs/λ standardized by the SAW wavelength λ ranges from about0.03 to about 0.45, which can be clearly seen in the subsequentexamples. In this case, Hs indicates the total thickness of the firstand the second SiO₂ insulating layers. With this range, the attenuationconstant can be considerably decreased compared to a SAW apparatuswithout a SiO₂ film, thereby achieving a reduction in the loss.

The ideal thickness of the IDTs 23 a and 23 b standardized by the SAWwavelength is different according to the material forming the IDTs 23 aand 23 b. If the IDTs are made of Au, the normalized thickness of theIDTs 23 a and 23 b is preferably from about 0.013 to about 0.030. If theAu film is too thin, the IDTs 23 a and 23 b exhibit a resistance.Accordingly, the normalized thickness of the IDTs 23 a and 23 b is, morepreferably, from about 0.021 to about 0.030.

According to the SAW apparatus of the second preferred embodiment of thepresent invention, the IDTs 23 a and 23 b are made of a metal having adensity higher than Al on the LiTaO₃ substrate 22, and the thickness ofthe IDTs 23 a and 23 b can be decreased. Thus, the SAW apparatusexhibits improved characteristics and also improves the TCF by theformation of the SiO₂ film 25. This is described in greater detail byspecific examples.

The electromechanical coupling coefficient KSAW, the reflectioncoefficient |ref|, and the attenuation constant (α) with respect to thenormalized thickness of IDTs when the IDTs were made of Al, Au, Ta, Ag,Cr, W, Cu, Zn, Mo, and Ni on a 36°-rotated Y-plate X-propagating LiTaO₃substrate having Euler angles (0°, 126°, 0°) are shown in FIGS. 16, 17,and 18, respectively. It should be noted that calculations were madeaccording to the method indicated in J. J. Chambell and W. R. Jones:IEEE Trans. Sonic & Ultrason. SU-15. p 209 (1968), assuming that theelectrodes were uniformly formed.

FIG. 16 shows that, in the IDT made of Al, the electromechanicalcoupling coefficient KSAW is about 0.27 when the normalized thicknessH/λ (H represents the thickness of the IDT and λ designates thewavelength) is about 0.10. In contrast, in the IDTs made of Au, Ta, Ag,Cr, W, Cu, Zn, Mo, and Ni, higher electromechanical couplingcoefficients KSAW are achieved when H/λ ranges from about 0.013 to about0.035. FIG. 18 reveals that, however, in the IDTs made of Au, Ta, Ag,Cr, W, Cu, Zn, Mo, and Ni, the attenuation constants α become verylarge, while, in the IDT made of Al, the attenuation constant α issubstantially 0 regardless of the normalized thickness H/λ.

FIG. 25 illustrates the relationship between the electromechanicalcoupling coefficient and Θ of the Euler angles (0°, Θ, 0°) when the AuIDT and the SiO₂ film are formed on a LiTaO₃ substrate having Eulerangles (0°, Θ, 0°). In this case, the normalized thickness of the IDTwas changed to about 0.022, 0.025, and 0.030, and the normalizedthickness Hs/λ of the SiO₂ film was changed to about 0.00 (without SiO₂film), 0.10, 0.20, 0.30, and 0.45.

FIG. 25 shows that the electromechanical coupling coefficient KSAWbecomes smaller as the thickness of the SiO₂ film is increased. It isnow assumed that the thickness of the IDT is decreased for suppressing acharacteristic deterioration caused by the formation of SiO₂ film, whichis described in detail below. FIG. 16 shows that the electromechanicalcoupling coefficient KSAW is decreased to about 0.245 when thenormalized thickness of the Al IDT is reduced to about 0.04 without theformation of SiO₂ film. If the normalized thickness of the Al IDT isreduced to about 0.04 with the formation of a SiO₂ film, theelectromechanical coupling coefficient KSAW becomes even smaller, whichmakes it difficult to achieve a wider band when the resulting SAWapparatus is put to practical use.

In contrast, as is seen from FIG. 25, when the IDT is formed of Au and aSiO₂ film is formed, the electromechanical coupling coefficient KSAW canbe increased to about 0.245 or greater by setting Θ of the Euler anglesto be about 128.5° or smaller even though the normalized thickness Hs/λof the SiO₂ film is about 0.45. When the normalized thickness of theSiO₂ SiO₂ SiO₂ film is about 0.30, the electromechanical couplingcoefficient KSAW can be increased to about 0.245 or greater by setting Θof the Euler angles to be about 132° or smaller. As discussed below,when Θ of the Euler angles is smaller than 115°, the attenuationconstant is increased, which makes it difficult put the SAW apparatus topractical use. Thus, preferably, a 25°-42°-rotated Y-plate X-propagatingLiTaO₃ substrate having Euler angles (0±3°, 115°-132°, 0±3°), and morepreferably, a 25°-38.5°-rotated Y-plate X-propagating LiTaO₃ substratehaving Euler angles (0±3°, 115°-128.5°, 0±3°) is used.

The temperature coefficient of frequency (TCF) of a 36°-rotated Y-plateX-propagating LiTaO₃ substrate having Euler angles (0°, 126°, 0°) is −30to −40 ppm/° C., which is not sufficient. In order to improve the TCF tobe about ±20 ppm/° C., an Au IDT was formed on a 36°-rotated Y-plateX-propagating LiTaO₃ substrate having Euler angles (0°, 126°, 0°), andthe thickness of the SiO₂ film was changed. In this case, the TCF withrespect to the normalized thickness of the SiO₂ film is shown in FIG.19. In FIG. 19, O indicates the ideal values, and × designates thevalues measured. In this case, the normalized thickness H/λ of the AuIDT is about 0.020.

FIG. 19 shows that the formation of the SiO₂ film improves the TCF, andin particular, when the normalized thickness Hs/λ of the SiO₂ film isabout 0.25, the TCF becomes substantially zero.

Also, by using two types of rotated Y-plate X-propagating LiTaO₃substrates, i.e., a substrate having a cut angle of 36° (Euler angles(0°, 126°, 0°)), and a substrate having a cut angle of 38° (Euler angles(0°, 128°, 0°)), the normalized thickness H/λ of an Au IDT and thenormalized thickness Hs/λ of a SiO₂ film were changed. The attenuationconstants α with respect to the normalized thickness of the SiO₂ filmare shown in FIGS. 20 and 21. FIGS. 20 and 21 show that the attenuationconstant α can be made smaller if the thickness of the SiO₂ film issuitably selected regardless of the thickness of the IDT. Morespecifically, as is seen from FIGS. 20 and 21, if the normalizedthickness Hs/λ of the SiO₂ film ranges from about 0.03 to about 0.45,and more preferably, from about 0.10 to about 0.35, the attenuationconstant α can be reduced to a minimal level regardless of theabove-described two types of Euler angles of the LiTaO₃ substrate andthe thickness of the Au IDT.

FIG. 17 shows that the use of an Au IDT achieves a sufficiently largereflection coefficient even with a small thickness of the IDT comparedto an Al IDT.

Thus, according to the results of FIGS. 16 through 21, when an Au IDThaving a normalized thickness H/λ of about 0.013 to about 0.030 isformed on a LiTaO₃ substrate, a large electromechanical couplingcoefficient can be achieved, and also, the attenuation coefficient α canbe reduced to a minimal level, and a sufficient reflection coefficientcan be implemented if the normalized thickness Hs/λ of the SiO₂ film ispreferably within the range from about 0.03 to about 0.45.

In the second preferred embodiment of the present invention, the SAWapparatus 11 was manufactured by forming an Au IDT having a normalizedthickness H/λ of about 0.020 and a SiO₂ film having a normalizedthickness Hs/λ of about 0.1 on a LiTaO₃ substrate having a cut angle of36° (Euler angles (0°, 126°, 0°)). The attenuation-vs.-frequencycharacteristic of the SAW apparatus 11 is indicated by the broken lineof FIG. 22. For comparison, the attenuation-vs.-frequency characteristicof the SAW apparatus 11 before the formation of the SiO₂ film is alsoindicated by the solid line of FIG. 22.

FIG. 22 shows that, because of the formation of the SiO₂ SiO₂ film, theinsertion loss is decreased even though the electromechanical couplingcoefficient is slightly reduced from about 0.30 to about 0.28.Accordingly, it has been proved that the attenuation constant α can bedecreased if the thickness of the SiO₂ film is set to theabove-described specific range.

After discovering the above-described fact, the present inventors formedone-port SAW resonators on an experimental basis by forming an Au IDThaving a normalized thickness of about 0.02 and a SiO₂ film on rotatedY-plate X-propagating LiTaO₃ substrates having different Euler angles.In this case, the normalized thickness of the SiO₂ film was changed toabout 0.10, 0.20, 0.30, and 0.45. The Q factors of the one-port SAWresonators are shown in FIG. 26.

Generally, as the Q factor of a resonator is increased, the sharpness ofthe filter characteristic of the resonator from the pass band to theattenuation range is increased. Accordingly, if a sharp filtercharacteristic is required, a greater Q factor is desirable. As is seenfrom FIG. 26, when the cut angle of the substrate is about 48° (Eulerangles of about (0°, 138°, 0°)), the Q factor becomes maximum, and whenthe cut angle ranges from about 42° to about 58° (Euler angles of aboutof (0°, 132°-148°, 0°)), the Q factor becomes comparatively largeregardless of the thickness of the SiO₂ film.

Accordingly, as is seen from FIG. 26, by forming a SAW resonator suchthat at least one IDT made of a metal having a density higher than Al isformed on a Y-plate LiTaO₃ substrate having a cut angle of about 42° toabout 58° (Euler angles of about (0°, 132°-148°, 0°)), and a SiO₂ filmis formed to cover the IDT on the LiTaO₃ substrate, a large Q factor canbe obtained. It is preferable that the cut angle of the LiTaO₃ substrateis about 46.5° to about 53° (Euler angles of about (0°, 136.5°-143°,0°)), as can be seen from FIG. 26.

In preferred embodiments the present invention, a contact layer may beformed on the top surface of the IDT. More specifically, as shown inFIG. 27A, an IDT 33 is formed on a LiTaO₃ substrate 32, and a contactlayer 34 may be formed on the top surface of the IDT 33. The contactlayer 34 is disposed between the IDT 33 and a SiO₂ film 35, so that itincreases the contact strength of the SiO₂ film 35 to the IDT 33. As thematerial for the contact layer 34, Pd or Al, or an alloy thereof may besuitably used. The contact layer 34 is not restricted to a metal, and apiezoelectric material, such as ZnO, or ceramics, such as Ta2O3 orAl2O3, may be used. The formation of the contact layer 34 increases thecontact strength between the IDT 33 and the SiO₂ film 35, therebypreventing the SiO₂ film 35 from peeling off.

The thickness of the contact layer 34 is preferably about 1% or less ofthe SAW wavelength so as to minimize the influence on the SAW by theformation of the contact layer 34. Although the contact layer 34 isformed only on the top surface of the IDT 33 in FIG. 27A, it may also beformed at the interface between the LiTaO₃ substrate 33 and the SiO₂film 35, as shown in FIG. 27B. Alternatively, as shown in FIG. 27C, thecontact layer 34 may also be formed, not only on the top surface of theIDT 33, but also on the side surfaces of the IDT 33.

As another configuration for improving the contact strength of the SiO₂film, a plurality of electrodes including bus bars and externallyconnecting pads other than IDTs may be laminated with an underlyingmetal layer formed of the same material as the IDT, and an upper metallayer made of Al or an Al alloy laminated with the underlying metallayer. For example, as an electrode film forming the reflectors 24 a and24 b shown in FIG. 15, an underlying metal layer made of the samematerial as the IDTs 23 a and 23 b and an Al film may be laminated onthe underlying metal layer. Accordingly, by providing an upper metallayer made of Al or an Al alloy, the contact strength of the SiO_(2 SiO)₂ film can be enhanced. Additionally, the cost of the electrode can bereduced, and the Al wedge bonding can also be enhanced.

The electrodes other than IDTs include, not only reflectors, bus bars,and externally connecting pads, but also wiring electrodes, which areformed if necessary. The Al alloy may include an Al—Ti alloy or anAl—Ni—Cr alloy by way of examples only.

The present inventors have confirmed that there is a certain range ofthickness of the SiO₂ film that minimizes the attenuation constant α aslong as an Au IDT is formed even when a Y-plate X-propagating LiTaO₃substrate having Euler angles other than the above-described angles isused. That is, if the normalized thickness Hs/λ of the SiO₂ film is setto be a specific range, the attenuation constant α can be reduced, as inthe above-described example. The relationship between the attenuationconstant α and the Euler angle Θ when the normalized thickness Hs/λ ofthe SiO₂ film was about 0.1 to about 0.45 are shown in FIGS. 28 through35. FIGS. 28 through 35 show that the Euler angle Θ that minimizes theattenuation constant α becomes smaller as the thickness of the SiO₂ filmis increased. Accordingly, even when a Y-plate X-propagating LiTaO₃substrate having Euler angles other than the above-described angles isused, it is possible to provide a SAW apparatus that exhibits a largeelectromechanical coupling coefficient and a large reflectioncoefficient and that reduces the TCF to one half of the known SAWapparatus if an Au IDT and a SiO₂ film are used. Preferable combinationsof the Euler angles, the thickness of the Au IDT, and the thickness ofthe SiO₂ film that achieve the above-described advantages are shown inTable 1 and Table 2.

TABLE 1 SiO₂ film Θ of Euler angles Au thickness thickness (0 ± 3°, Θ, 0± 3°) H/λ Hs/λ 120.0° ≦ Θ < 123.0° 0.013-0.018 0.15-0.45 123.0° ≦ Θ <124.5° 0.013-0.022 0.10-0.40 124.5° ≦ Θ < 125.5° 0.013-0.025 0.07-0.40125.5° ≦ Θ < 127.5° 0.013-0.025 0.06-0.40 127.5° ≦ Θ < 129.0°0.013-0.028 0.04-0.40 129.0° ≦ Θ < 130.0° 0.017-0.030 0.03-0.42 130.0° ≦Θ < 131.5° 0.017-0.030 0.03-0.42 131.5° ≦ Θ < 133.0° 0.018-0.0280.05-0.33 133.0° ≦ Θ < 135.0° 0.018-0.030 0.05-0.30 135.0° ≦ Θ < 137.0°0.019-0.032 0.05-0.25 137.0° ≦ Θ ≦ 140.0° 0.019-0.032 0.05-0.25

TABLE 2 SiO₂ film Θ of Euler angles Au thickness thickness (0 ± 3°, Θ, 0± 3°) H/λ Hs/λ 129.0° ≦ Θ < 130.0° 0.022-0.028 0.04-0.40 130.0° ≦ Θ <131.5° 0.022-0.028 0.04-0.40 131.5° ≦ Θ < 133.0° 0.022-0.028 0.05-0.33133.0° ≦ Θ < 135.0° 0.022-0.030 0.05-0.30 135.0° ≦ Θ < 137.0°0.022-0.032 0.05-0.25 137.0° ≦ Θ ≦ 140.0° 0.022-0.032 0.05-0.25

Euler angle Θ may sometimes deviate from the desired angle by −2° to+4°. This deviation is caused by the fact that calculations were made inthis preferred embodiment assuming that a metallic film was formed onthe entire surface of the substrate, and there may be some errors withinthe above range in actual SAW apparatuses.

When manufacturing the SAW apparatus according to the preferredembodiments of the present invention, it is preferable that an IDTprimarily including Au is formed on a rotated Y-plate X-propagatingLiTaO₃ substrate. In this state, the frequency of the SAW apparatus isadjusted. Then, a SiO₂ film, having a thickness reduces the attenuationconstant α, is formed. This is explained below with reference to FIGS.23 and 24. Au IDTs having different thickness values and SiO₂ filmshaving different thickness values were formed on a 36°-rotated Y-plateX-propagating LiTaO₃ substrate (Euler angles (0°, 126°, 0°)). FIG. 23illustrates a change in the acoustic velocity of a leaky SAW withrespect to the thickness of the IDT. FIG. 24 illustrates a change in theacoustic velocity of a leaky SAW with respect to the thickness of theSiO₂ film. FIGS. 23 and 24 show that a change in the acoustic velocityof the SAW is much larger when the thickness of the IDT is varied thanwhen the thickness of the SiO₂ film is varied. Accordingly, it isdesirable that the frequency is adjusted before the formation of theSiO₂ film. For example, it is desirable that the frequency is adjustedafter an Au IDT is formed by laser etching or ion etching. Morepreferably, the normalized thickness of the Au IDT ranges from about0.015 to about 0.030. In this case, a change in the acoustic velocity bya variation of a SiO₂ film is reduced, and a frequency fluctuation dueto a variation of the SiO₂ film is decreased.

Θ of the Euler angles may sometimes deviate from the desired angle byabout −2° to about +4°. This deviation is caused by the fact thatcalculations were made in this preferred embodiment assuming that ametallic film was formed on the entire surface of the substrate, andthere may be some errors within the above range in actual SAWapparatuses.

When manufacturing SAW apparatuses, although φ and ψ of the Euler anglesdeviate from 0° by ±3°, substantially the same characteristic as thatwhen φ and ψ are 0° can be obtained.

A SAW apparatus of a third preferred embodiment of the present inventionis described below. The SAW apparatus of the third preferred embodimentof the present invention is similar to the SAW apparatus 21 of thesecond preferred embodiment of the present invention shown in FIG. 15,except that the IDTs 23 a and 23 b are preferably made of Ag.

As stated below, when the IDTs 23 a and 23 b are made of Ag, thethickness H/λ of the IDTs 23 a and 23 b standardized by the SAWwavelength λ is preferably from about 0.01 to about 0.08.

According to the SAW apparatus of the third preferred embodiment of thepresent invention, the IDTs 23 a and 23 b are made of Ag on the LiTaO₃substrate 22 and the thickness of the IDTs 23 a and 23 b can bedecreased. Because the LiTaO₃ substrate is used, the attenuationconstant can be considerably decreased, thereby achieving low insertionloss. By the formation of the SiO₂ film 25, a high level of temperaturecoefficient of frequency (TCP) is achieved. This is described in detailbelow by way of specific examples.

SAWs propagating in a LiTaO₃ substrate include, not only Rayleigh wave,but also leaky SAW (LSAW). Although the LSAW has a higher acousticvelocity and a greater electromechanical coupling coefficient than theRayleigh wave, it propagates while radiating energy in the substrate.Accordingly, the LSAW causes attenuation which results in the insertionloss.

FIG. 36 illustrates the relationship between the electromechanicalcoupling coefficient KSAW and the normalized thickness H/λ of an Ag IDTon a 36°-rotated Y-plate X-propagating LiTaO₃ substrate (having Eulerangles (0°, 126°, 0°)). It should be noted that λ represents thewavelength at the center frequency of the SAW apparatus.

FIG. 36 shows that, when the thickness H/λ of the Ag film ranges fromabout 0.01 to about 0.08, the electromechanical coupling coefficientKSAW becomes about 1.5 times or greater than the electromechanicalcoupling coefficient of a SAW apparatus without an Ag film (H/λ=0). Whenthe thickness H/λ of the Ag film ranges from about 0.02 to about 0.06,the electromechanical coupling coefficient KSAW becomes about 1.7 timesor greater than the electromechanical coupling coefficient of a SAWapparatus without an Ag film. When the thickness H/λ of the Ag filmranges from about 0.03 to about 0.05, the electromechanical couplingcoefficient KSAW becomes about 1.8 times or greater than theelectromechanical coupling coefficient of a SAW apparatus without an Agfilm.

If the thickness H/λ of the Ag film exceeds about 0.08, it becomesdifficult to form an Ag IDT. Accordingly, in order to obtain a largeelectromechanical coupling coefficient without a difficulty in formingan Ag IDT, the thickness of the Ag IDT is desirably from about 0.01 toabout 0.08, and more preferably, from about 0.02 to about 0.06, andfurther preferably, about 0.03 to about 0.05.

The relationship between the TCF and the thickness Hs/λ of a SiO₂ filmformed on a LiTaO₃ substrate is shown in FIG. 37. FIG. 37 shows theresults obtained when three types of LiTaO₃ substrates having Eulerangles (0°, 113°, 0°), (0°, 126°, 0°), (0°, 129°, 0°) were used. In thisexample, an electrode is not formed.

FIG. 37 reveals that the TCF ranges from about −20 to about +20 ppm/° C.when the thickness Hs/λ of the SiO₂ film is from about 0.15 to about0.45, regardless of whether the angle Θ is 113°, 126°, or 129°. Becauseof the time it takes to form a SiO₂ film, the thickness Hs/λ of the SiO2film is desirably about 0.15 to about 0.40.

FIG. 38 illustrates a change in the attenuation constant α when Agelectrodes having a normalized thickness H/λ of about 0.10 or smallerand SiO₂ films having a normalized thickness Hs/λ of 0 to about 0.5 wereformed on a LiTaO₃ substrate having Euler angles (0°, 120°, 0°). FIG. 38shows that the attenuation constant α is small when the thickness Hs/λof the SiO₂ film is about 0.2 to about 0.4, and when the thickness H/λof the Ag film is about 0.01 to about 0.10.

FIG. 39 illustrates a change in the attenuation constant α when Agelectrodes having a normalized thickness H/λ of 0 to about 0.10 and SiO₂films having a normalized thickness Hs/λ of 0 to about 0.5 were formedon a LiTaO₃ substrate having Euler angles (0°, 140°, 0°). As is seenfrom FIG. 39, when Θ is 140°, the attenuation constant α becomes largeras the thickness of the SiO₂ film is increases, as described above, andas the normalized thickness of the Ag film decreases, especially whenthe normalized thickness of the Ag film is about 0.06 or smaller.

That is, in order to achieve an improved TCF, a large electromechanicalcoupling coefficient, and a small attenuation constant, it is necessaryto suitably combine the cut angle of a LiTaO₃ substrate, the thicknessof a SiO₂ film, and the thickness of an Ag film.

FIGS. 40 through 47 illustrate the relationship between the attenuationconstant α and Θ of the Euler angles when Ag films having a normalizedthickness H/λ of about 0.1 or smaller were formed on a LiTaO₃ substrateand when the normalized thickness Hs/λ of the SiO₂ film was changed toabout 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, and 0.45, respectively.

As is seen from FIGS. 40 through 47, by setting the thickness of the Agfilm to be about 0.01 to about 0.08 and by selecting any of thefollowing combinations of the SiO₂ film and Θ of the Euler angles shownin the center column of table Table 3, it is possible to implement ahigh level of TCF, a large electromechanical coupling coefficient, and asmall attenuation constant α. It is more preferable that the Eulerangles on the right side of table are selected. In which case, superiorcharacteristics are obtained.

TABLE 3 Ag thickness H/λ: about 0.01 to about 0.08 SiO₂ thickness Eulerangles of More preferable Hs/λ LiTaO₃ (°) Euler angles (°) 0.15-0.18 0 ±3, 117-137, 0 ± 3 0 ± 3, 120-135, 0 ± 3 0.18-0.23 0 ± 3, 117-136, 0 ± 30 ± 3, 118-133, 0 ± 3 0.23-0.28 0 ± 3, 115-135, 0 ± 3 0 ± 3, 117-133, 0± 3 0.28-0.33 0 ± 3, 113-133, 0 ± 3 0 ± 3, 115-132, 0 ± 3 0.33-0.38 0 ±3, 113-135, 0 ± 3 0 ± 3, 115-133, 0 ± 3 0.38-0.40 0 ± 3, 113-132, 0 ± 30 ± 3, 115-130, 0 ± 3

When the normalized thickness H/λ of the Ag film is about 0.02 to about0.06, any of the following combinations shown in Table 4 of thenormalized thickness of the SiO₂ film and Θ of the Euler angles in thecenter column, and more preferable Euler angles on the right side ofTable 4, in which case superior characteristics are obtained, can beselected.

TABLE 4 Ag thickness H/λ: about 0.02 to about 0.06 SiO₂ thickness Eulerangles of More preferable (Hs/λ) LiTaO₃ (°) Euler angles (°) 0.15-0.18 0± 3, 120-133, 0 ± 3 0 ± 3, 122-130, 0 ± 3 0.18-0.23 0 ± 3, 120-137, 0 ±3 0 ± 3, 122-136, 0 ± 3 0.23-0.28 0 ± 3, 120-135, 0 ± 3 0 ± 3, 122-133,0 ± 3 0.28-0.33 0 ± 3, 118-135, 0 ± 3 0 ± 3, 120-133, 0 ± 3 0.33-0.38 0± 3, 115-133, 0 ± 3 0 ± 3, 117-130, 0 ± 3 0.38-0.40 0 ± 3, 113-130, 0 ±3 0 ± 3, 115-128, 0 ± 3

When the standard thickness H/λ of the Ag film is about 0.03 to about0.05, any of the following combinations shown in Table 5 of thethickness of the SiO₂ film and Θ of the Euler angles in the centercolumn, and more preferable Euler angles on the right side of Table 5,in which case superior results are obtained, can be selected.

TABLE 5 Ag thickness H/λ: about 0.03 to about 0.05 SiO₂ thickness Eulerangles of More preferable Hs/λ LiTaO₃ (°) Euler angles (°) 0.15-0.18 0 ±3, 122-142, 0 ± 3 0 ± 3, 123-140, 0 ± 3 0.18-0.23 0 ± 3, 120-140, 0 ± 30 ± 3, 122-137, 0 ± 3 0.23-0.28 0 ± 3, 117-138, 0 ± 3 0 ± 3, 120-135, 0± 3 0.28-0.33 0 ± 3, 116-136, 0 ± 3 0 ± 3, 118-134, 0 ± 3 0.33-0.38 0 ±3, 114-135, 0 ± 3 0 ± 3, 117-133, 0 ± 3 0.38-0.40 0 ± 3, 113-130, 0 ± 30 ± 3, 115-128, 0 ± 3

In preferred embodiments of the present invention, the IDT may be madeof only Ag. Alternatively, the IDT may be made of an Ag alloy or alaminated electrode of Ag and another metal, as long as such an alloy ora laminated electrode primarily comprises Ag. In this case, it ispreferably that Ag constitutes about 80% by weight of the total IDT.Accordingly, an Al thin film or a Ti thin film may be formed as anunderlying layer of the Ag IDT. In this case, it is preferable that Agconstitutes about 80% by weight of the total of the underlying layer andthe IDT.

In the above-described example, a LiTaO₃ substrate having Euler angles(0°, Θ, 0°) was used, and normally, there is a variation of 0±3° in φand ψ. However, even in a LiTaO₃ substrate having such a variation,i.e., (0±3°, 113°-142°, 0±3°), advantages of the preferred embodimentsof the present invention can be achieved.

Euler angle Θ may sometimes deviate from the desired angle by about −2°to about +40°. This deviation is generated caused by the fact thatcalculations were made in this preferred embodiment assuming that ametallic film was formed on the entire surface of the substrate, andthere may be some errors within the above range in actual SAWapparatuses.

A SAW apparatus of a fourth preferred embodiment of the presentinvention is described below. The SAW apparatus of the fourth preferredembodiment of the present invention is similar to the SAW apparatus 21of the second preferred embodiment of the present invention shown inFIG. 15, except that the IDTs 23 a and 23 b are made of Cu. Because theelectrodes are made of Cu having a higher density than Al, theelectromechanical coupling coefficient and the reflection coefficientare improved.

FIG. 58 illustrates the relationship between the reflection coefficientof a Cu electrode and that of an Al electrode and the thickness of thecorresponding electrode when the normalized thickness of a SiO₂ film isabout 0.20.

FIG. 58 shows that the reflection coefficient per electrode finger canbe increased when a Cu electrode was used rather than an Al electrode.In this case, the number of electrode fingers can be decreased. Thus,the size of the reflectors can be reduced, and accordingly, the overallsize of the resulting SAW apparatus can be reduced.

As discussed below, the thickness H/λ of the IDTs 23 a and 23 bstandardized by the wavelength λ is preferably from about 0.01 to about0.08.

FIG. 48 illustrates a change in the attenuation constant α when Cuelectrodes having a normalized thickness of H/λ of about 0.10 or smallerand SiO₂ films having a normalized thickness Hs/λ of 0 to about 0.5 wereformed on a LiTaO₃ substrate having Euler angles (0°, 120°, 0°). FIG. 48shows that the attenuation constant α is small when the thickness Hs/λof the SiO₂ film is about 0.2 to about 0.4 and when the thickness H/λ ofthe Cu film is about 0.01 to about 0.10.

FIG. 49 illustrates a change in the attenuation constant α when Cuelectrodes having a normalized thickness of H/λ of 0 to about 0.10 andSiO2 films having a normalized thickness Hs/λ of 0 to about 0.5 wereformed on a LiTaO₃ substrate having Euler angles (0°, 135°, 0°). As isseen from FIG. 49, when Θ is 135°, the attenuation constant α becomeslarger as the normalized thickness of the Cu film decreases and thenormalized thickness of the SiO₂ film increases.

Accordingly, in order to achieve an improved TCF, a largeelectromechanical coupling coefficient, and a small attenuationconstant, it is necessary to suitably combine the cut angles of a LiTaO₃substrate, the thickness of a SiO₂ film, and the thickness of a Cuelectrode.

FIGS. 50 through 57 illustrate the relationship between the attenuationconstant α and Θ of the Euler angles when the Cu films having anormalized thickness H/λ of about 0.1 or smaller were formed on a LiTaO₃substrate and when the normalized thickness Hs/λ of the SiO₂ SiO₂ filmwas changed to about 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, and 0.45,respectively.

As is seen from FIGS. 50 through 57, by setting the thickness H/λ of theCu film to be about 0.01 to about 0.08 and by selecting any of thefollowing combinations of the SiO₂ SiO₂ film and Θ of the Euler anglesshown in the center column of Table 6, it is possible to implement animproved TCF (±20 ppm/° C.), a large electromechanical couplingcoefficient, and a small attenuation constant α. More preferable, Eulerangles shown on the right side of Table 6 are selected. In which case,superior characteristics are obtained.

TABLE 6 Ag thickness H/λ: about 0.02 to about 0.06 SiO₂ thickness Eulerangles of More preferable Hs/λ LiTaO₃ (°) Euler angles (°) 0.15-0.18 0 ±3, 117-137, 0 ± 3 0 ± 3, 120-135, 0 ± 3 0.18-0.23 0 ± 3, 117-136, 0 ± 30 ± 3, 118-133, 0 ± 3 0.23-0.28 0 ± 3, 115-135, 0 ± 3 0 ± 3, 117-133, 0± 3 0.28-0.33 0 ± 3, 113-133, 0 ± 3 0 ± 3, 115-132, 0 ± 3 0.33-0.38 0 ±3, 113-135, 0 ± 3 0 ± 3, 115-133, 0 ± 3 0.38-0.40 0 ± 3, 113-132, 0 ± 30 ± 3, 115-130, 0 ± 3

As can be inferred from the electromechanical coupling coefficientK_(SAW) when the Au electrode was used shown in FIG. 25, theelectromechanical coupling coefficient K_(SAW) is considerably increasedwhen Θ of the Euler angle is about 125° or smaller. Accordingly, it ismore preferable that the combinations of the normalized thickness Hs/λof the SiO₂ film and the Euler angles shown in Table 7 are selected.

TABLE 7 SiO₂ thickness Euler angles of Hs/λ LiTaO₃ (°) 0.15-0.18 0 ± 3,117-125, 0 ± 3 0.18-0.23 0 ± 3, 117-125, 0 ± 3 0.23-0.28 0 ± 3, 115-125,0 ± 3 0.28-0.33 0 ± 3, 113-125, 0 ± 3 0.33-0.38 0 ± 3, 113-125, 0 ± 30.38-0.40 0 ± 3, 113-125, 0 ± 3

The Euler angle Θmin that reduces the attenuation constant α tosubstantially 0 or minimizes the attenuation constant α with respect tothe normalized thickness Hs/λ of the SiO₂ film and the normalizedthickness H/λ of the Cu film was determined from the results of FIGS. 48through 56. Such an Euler angle Θmin is shown in FIG. 59.

By approximating the curves shown in FIG. 59 with a cubic polynomialwhen the normalized thickness H/λ of the Cu film is approximately 0,0.02, 0.04, 0.06, and 0.08, the following equations A through E werefound:

-   -   (a) When 0<H/λ≦0.01        Θ_(min)=−139.713×Hs ³+43.07132×Hs ²−20.568011×Hs+125.8314  A    -   (b) When 0.01<H/λ≦0.03        Θ_(min)=−139.660×Hs ³+46.02985×Hs ²−21.141500×Hs+127.4181  B    -   (c) When 0.03<H/λ≦0.05        Θ_(min)=−139.607×Hs ³+48.98838×Hs ²−21.714900×Hs+129.0048  C    -   (d) When 0.05<H/λ≦0.07        Θ_(min)=−112.068×Hs ³+39.60355×Hs ²−21.186000×Hs+129.9397  D    -   (e) When 0.07<H/λ≦0.09        Θ_(min)=−126.954×Hs ³+67.40488×Hs ²−29.432000×Hs+131.5686  E

Accordingly, it is preferable that Θ of the Euler angles (0±3°, Θ, 0±3°)is Θmin determined by the above-described equations A through E.However, when Θmin−2°<Θ≦Θmin+2°, the attenuation constant can beeffectively decreased.

In preferred embodiments of the present invention, the IDT may be madeof only Cu. Alternatively, the IDT may be made of a Cu alloy or alaminated electrode of Cu and another metal, as long as such an alloy ora laminated electrode primarily comprises Cu. More specifically, the IDTprimarily including Cu preferably satisfies the following condition whenthe average density of the electrode is indicated by ρ (average):ρ(Cu)×0.7≦ρ(average)≦ρ(Cu)×1.3,i.e., 6.25 g/cm³≦ρ(average)≦11.6 g/cm³.

An upper layer or an underlying layer made of a metal having a densityhigher than Al, such as W, Ta, Au, Pt, Ag, or Cr, may be laminated onthe Cu electrode so that ρ (average) satisfies the above-describedcondition. In this case, advantages similar to those obtained by asingle Cu layer can be achieved.

Θ of the Euler angles may sometimes deviate from the desired angle byabout −2° to about +4°. This deviation is caused by the fact thatcalculations were made in this preferred embodiment assuming that ametallic film was formed on the entire surface of the substrate, andthere may be some errors within the above range in actual SAWapparatuses.

When manufacturing SAW apparatuses, there is a variation of about 0±3°in φ and ψ of the Euler angles. However, substantially the samecharacteristic as that when φ and ψ are 0° can be obtained.

A SAW apparatus of a fifth preferred embodiment of the present inventionis described below. The SAW apparatus of the fifth preferred embodimentof the present invention is similar to the SAW apparatus 21 of thesecond preferred embodiment of the present invention shown in FIG. 15,except that the IDTs 23 a and 23 b and the reflectors 24 a and 24 b aremade of tungsten (W). The normalized thickness H/λ of the IDTs is about0.0025 to about 0.06.

As the piezoelectric substrate 22, a 22°-48°-rotated Y-plateX-propagating LiTaO₃ substrate having Euler angles (0°, 112°-138°, 0°)was used.

In the fifth preferred embodiment of the present invention, because the22°-48°-rotated Y-plate X-propagating LiTaO₃ substrate 22, the IDTs 23 aand 23 b made of tungsten having a thickness H/λ of about 0.0025 toabout 0.06, and the SiO₂ film 25 having a thickness Hs/λ of about 0.10to about 0.40 were used, it is possible to provide a SAW apparatus thathas an improved TCF, a large electromechanical coupling coefficientKSAW, and a small propagation loss. The fifth preferred embodiment ofthe present invention is described in detail below by way of a specificexample.

FIGS. 60 and 61 illustrate a change in the attenuation constant α whentungsten IDTs having different thickness values and SiO₂ films havingdifferent thickness values were formed on a LiTaO₃ substrate havingEuler angles (0°, 120°, 0°) and a LiTaO₃ substrate having Euler angles(0°, 140°, 0°), respectively.

As is seen from FIG. 60, when Θ is 120°, the attenuation constant α issmall when the thickness Hs/λ of the SiO₂ film is about 0.1 to about 0.4and the thickness H/λ of the tungsten electrode is about 0.0 to about0.10. As is seen from FIG. 61, when Θ is 140°, the attenuation constantα increases as the thickness H/λ of the tungsten electrode changes from0.0 to about 0.10, regardless of the thickness Hs/λ of the SiO₂ film.

In order to reduce the TCF to be between about −20 ppm/° C. and about+20 ppm/° C., to achieve a large electromechanical coupling coefficient,and to decrease the attenuation constant, the Euler angles of the LiTaO₃substrate, the thickness of the SiO₂ film, and the thickness of thetungsten electrode, must be considered.

FIGS. 62 through 65 illustrate the relationship between the attenuationconstant α and Θ of the Euler angles when the normalized thickness Hs/λof the SiO₂ film and the normalized thickness H/λ of the tungstenelectrode were changed.

As is seen from FIGS. 62 through 65, optimal combinations of thenormalized thickness of the SiO₂ film and the Euler angle Θ, when thenormalized thickness H/λ of the tungsten electrode is about 0.012 toabout 0.053 and, more preferably, is about 0.015 to about 0.042, can beselected from the combinations shown in Table 8 and Table 9,respectively. Euler angle Θ shown in Table 8 and Table 9 may vary byabout −2° to about +4° due to a variation in the electrode finger widthof the tungsten electrode or a variation in the single crystalsubstrate. The thickness values which are not shown in FIGS. 62 through65 are determined by the proportional distribution.

TABLE 8 Tungston thickness H/λ: about 0.012 to about 0.053 SiO₂thickness Euler angles of More preferable Hs/λ LiTaO₃ (°) Euler angles(°) 0.10-0.15 0 ± 3, 114.2-138.0, 0 ± 3 0 ± 3, 117.7-134.0, 0 ± 30.15-0.20 0 ± 3, 113.0-137.8, 0 ± 3 0 ± 3, 117.0-133.5, 0 ± 3 0.20-0.300 ± 3, 113.0-137.5, 0 ± 3 0 ± 3, 116.5-133.0, 0 ± 3 0.30-0.35 0 ± 3,112.7-137.0, 0 ± 3 0 ± 3, 116.5-133.0, 0 ± 3 0.35-0.40 0 ± 3,112.5-136.0, 0 ± 3 0 ± 3, 116.5-132.3, 0 ± 3

TABLE 9 Tungston thickness H/λ: about 0.015 to about 0.042 SiO₂thickness Euler angles of More preferable (Hs/λ) LiTaO₃ (°) Euler angles(°) 0.10-0.15 0 ± 3, 114.3-138.0, 0 ± 3 0 ± 3, 117.7-133.5, 0 ± 30.15-0.20 0 ± 3, 113.0-137.5, 0 ± 3 0 ± 3, 117.7-133.5, 0 ± 3 0.20-0.300 ± 3, 112.5-137.0, 0 ± 3 0 ± 3, 117.0-132.5, 0 ± 3 0.30-0.35 0 ± 3,112.2-136.5, 0 ± 3 0 ± 3, 116.8-132.5, 0 ± 3 0.35-0.40 0 ± 3,112.0-135.3, 0 ± 3 0 ± 3, 116.0-131.5, 0 ± 3

When the normalized thickness H/λ of the tungsten electrode is about0.012 to about 0.053, as indicated in Table 8, the normalized thicknessHs/λ of the SiO₂ SiO₂ film is preferably about 0.1 to about 0.4 in orderto set the range of the TCF to be between about −20 ppm/° C. and about+20 ppm/° C. In this case, Euler angle Θ of the LiTaO₃ substrate ispreferably between about 112° and about 138° (corresponding to therotation angle of about 20° to about 50°), More preferably, the Eulerangles indicated on the right side of Table 8 are selected.

Similarly, when the normalized thickness H/λ of the tungsten electrodeis about 0.015 to about 0.042, as indicated in Table 9, the normalizedthickness Hs/λ of the SiO₂ film is preferably about 0.1 to about 0.4 inorder to set the range of the TCF to be between about − and about +20ppm/° C. In this case, Θ of the Euler angles of the LiTaO₃ substrate ispreferably between about 112° and about 138°. More preferably, the Eulerangles indicated on the right side of Table 9 are selected.

The Euler angles of LiTaO₃ shown in Table 8 and Table 9 were selected sothat the attenuation constant becomes about 0.05 or lower. The morepreferable Euler angles shown in Table 8 and Table 9 were selected sothat the attenuation constant becomes about 0.025 or lower. Therelationships between the Hs/λ of the SiO₂ film and the Euler anglesshown in Table 8 and Table 9 when the thickness H/λ of the tungstenelectrode is approximately 0.012, 0.015, 0.042, and 0.053 weredetermined in terms of the thickness H/λ of the tungsten electrode shownin FIGS. 62 through 65.

When manufacturing the SAW apparatus of this preferred embodiment, it ispreferable that an IDT primarily including tungsten is formed on arotated Y-plate X-propagating LiTaO₃ substrate. Then, the frequency isadjusted. Then, a SiO₂ film having a thickness that can reduce theattenuation constant α is formed. This is explained below with referenceto FIGS. 66 and 67. Tungsten IDTs having different thickness values andSiO₂ films having different thickness values were formed on a rotatedY-plate X-propagating LiTaO₃ substrate (Euler angles (0°, 126°, 0°)).FIG. 66 illustrates a change in the acoustic velocity of a leaky SAWwith respect to the thickness of the SiO₂ film. FIG. 67 illustrates achange in the acoustic velocity of a leaky SAW with respect to thethickness of the tungsten electrode. FIGS. 66 and 67 show that a changein the acoustic velocity of the SAW is much larger when the thickness ofthe tungsten IDT is varied than when the thickness of the SiO₂ film isvaried. Accordingly, it is desirable that the frequency is adjustedbefore the formation of the SiO₂ film. It is desirable that thefrequency is adjusted after a tungsten IDT is formed by laser etching orion etching.

In this preferred embodiment, a 22°-48°-rotated Y-plate X-propagatingLiTaO₃ substrate having Euler angles (0°, 112°-138°, 0°), a tungsten IDThaving a thickness H/λ of about 0.0025 to about 0.06, and a SiO₂ filmhaving a thickness Hs/λ of about 0.10 to about 0.40 are used. The numberand the structure of IDTs are not particularly restricted. That is, thepresent invention can be applied to, not only the SAW apparatus shown inFIG. 15, but also various types of SAW resonators and SAW filters aslong as the above-described conditions are satisfied.

Euler angles Θ may sometimes deviate from the desired angle by about −2°to about +4°. This deviation is caused by the fact that calculationswere made in this embodiment assuming that a metallic film was formed onthe entire surface of the substrate, and there may be some errors withinthe above range in actual SAW apparatuses.

When manufacturing SAW apparatuses, although φ and ψ of the Euler anglesdeviate from 0° by ±3°, substantially the same characteristic as thatwhen φ and ψ are 0° can be obtained.

A SAW apparatus of a sixth preferred embodiment of the present inventionis described below. The SAW apparatus of the sixth preferred embodimentof the present invention is similar to the SAW apparatus 21 of thesecond preferred embodiment of the present invention shown in FIG. 15.However, as the piezoelectric substrate 22, a 14°-58°-rotated Y-plateX-propagating LiTaO₃ substrate having Euler angles (0°, 104°-148°, 0°)was used, and IDTs made of tantalum (Ta) having the thickness H/λ ofabout 0.004 to about 0.055 were used.

In the sixth preferred embodiment, because a 14°-58°-rotated Y-plateX-propagating LiTaO₃ substrate 22 having Euler angles (0°, 104°-148°,0°), IDTs 23 a and 23 b made of tantalum having a thickness H/λ of about0.004 to about 0.055, and a SiO₂ film 25 having a thickness Hs/λ ofabout 0.10 to about 0.40 were used, it is possible to provide a SAWapparatus that has an improved TCF, a large electromechanical couplingcoefficient KSAW, and a small propagation loss. The sixth preferredembodiment of the present invention is described in detail below by wayof a specific example.

FIGS. 68 and 69 illustrate a change in the attenuation constant α whentantalum IDTs having different thickness values and SiO₂ films havingdifferent thickness values were formed on a LiTaO₃ substrate havingEuler angles (0°, 120°, 0°) and a LiTaO₃ substrate having Euler angles(0°, 140°, 0°).

As is seen from FIG. 68, when Θ is 120°, the attenuation constant α issmall when the thickness Hs/λ of the SiO₂ film is about 0.1 to about 0.4and when the thickness H/λ of the tantalum electrode is about 0.0 toabout 0.1. In contrast, as is seen from FIG. 69, when Θ is 140°, theattenuation constant α is large when the thickness H/λ of the tantalumelectrode is about 0.0 to about 0.06 regardless of the thickness Hs/λ ofthe SiO₂ film.

In order to decrease the absolute value of the TCF, to achieve a largeelectromechanical coupling coefficient, and to decrease the attenuationconstant, the Euler angles of the LiTaO₃ substrate, the thickness of theSiO₂ film, and the thickness of the tantalum electrode must beconsidered.

FIGS. 70 and 73 illustrate relationships between the attenuationconstant α and the Euler angle Θ when the normalized thickness Hs/λ ofthe SiO₂ film and the normalized thickness H/λ of the tantalum electrodewere changed.

As is seen from FIGS. 70 through 73, optimal combinations of thenormalized thickness Hs/λ of the SiO₂ film and the Euler angle Θ, whenthe thickness H/λ of the tantalum electrode is about 0.01 to about0.055, more preferably between about 0.016 to about 0.045, can beselected from the combinations shown in Table 10 and Table 11,respectively. Euler angle Θ shown in Table 10 and Table 11 may vary byabout −2° to about +4° because of a variation in the electrode fingerwidth of the tantalum electrode or a variation in the single crystalsubstrate.

TABLE 10 Tantalum thickness H/λ: about 0.01 to about 0.055 SiO₂thickness Euler angles of More preferable Hs/λ LiTaO₃ (°) Euler angles(°) 0.10-0.15 0 ± 3, 110.5-148.0, 0 ± 3 0 ± 3, 116.0-143.0, 0 ± 30.15-0.20 0 ± 3, 108.0-147.5, 0 ± 3 0 ± 3, 115.0-141.5, 0 ± 3 0.20-0.300 ± 3, 105.0-148.0, 0 ± 3 0 ± 3, 111.0-139.0, 0 ± 3 0.30-0.35 0 ± 3,104.5-148.0, 0 ± 3 0 ± 3, 111.0-139.0, 0 ± 3 0.35-0.40 0 ± 3,104.0-145.0, 0 ± 3 0 ± 3, 110.0-138.5, 0 ± 3

TABLE 11 Tantalum thickness H/λ: about 0.016 to about 0.045 SiO₂thickness Euler angles of LiTaO₃ More preferable Euler (Hs/λ) (°) angles(°) 0.10-0.15 0 ± 3, 113.0-144.0, 0 ± 3 0 ± 3, 118.0-140.0, 0 ± 30.15-0.20 0 ± 3, 111.0-144.0, 0 ± 3 0 ± 3, 117.0-139.5, 0 ± 3 0.20-0.300 ± 3, 108.0-144.0, 0 ± 3 0 ± 3, 113.0-139.0, 0 ± 3 0.30-0.35 0 ± 3,107.5-143.0, 0 ± 3 0 ± 3, 112.5-137.0, 0 ± 3 0.35-0.40 0 ± 3,107.0-140.5, 0 ± 3 0 ± 3, 112.0-135.5, 0 ± 3

When the thickness H/λ of the tantalum electrode is about 0.01 to about0.055, as indicated in Table 10, the thickness Hs/λ of the SiO₂ film ispreferably about 0.1 to about 0.4 in order to set the range of the TCFto between about −20 ppm/°C. and about +20 ppm/°C. In this case, Eulerangle Θ of the LiTaO₃ substrate are preferably between about 104° andabout 148° (corresponding to the rotation angle of about 14° to about58°), and more preferably, the Euler angles indicated on the right sideof Table 10 are selected according to the thickness Hs/λ of the SiO₂film.

Similarly, when the thickness H/λ of the tantalum electrode is about0.016 to about 0.045, as indicated in Table 11, the thickness Hs/λ ofthe SiO₂ film is preferably about 0.1 to about 0.4 in order to improvethe TCF. In this case, Euler angle Θ of the LiTaO₃ substrate ispreferably between about 107° and about 144°, and more preferably, theEuler angles indicated on the right side of Table 11 are selectedaccording to the thickness of the SiO₂ film.

The Euler angles of LiTaO₃ shown in Table 10 and Table 11 were selectedso that the attenuation constant becomes about 0.05 or lower. The morepreferable Euler angles shown in Table 10 and Table 11 were selected sothat the attenuation constant becomes about 0.025 or lower. Therelationships between the Hs/λ of the SiO₂ film and the Euler anglesshown in Table 10 and Table 11 when the thickness H/λ of the tantalumelectrode is about 0.012, 0.015, 0.042, and 0.053 were determined interms of the thickness H/λ of the tantalum electrode shown in FIGS. 70through 73.

When manufacturing the SAW apparatus of this preferred embodiment, it ispreferable that an IDT primarily including tantalum is formed on arotated Y-plate X-propagating LiTaO₃ substrate. Then, the frequency isadjusted Then, a SiO₂ film having a thickness that reduces theattenuation constant α is formed. This is explained below with referenceto FIGS. 74 and 75. Tantalum IDTs having different thickness values andSiO₂ films having different thickness values were formed on a rotatedY-plate X-propagating LiTaO₃ substrate (Euler angles (0°, 126°, 0°).FIG. 74 illustrates a change in the acoustic velocity of a leaky SAWwith respect to the thickness of the SiO₂ film. FIG. 75 illustrates achange in the acoustic velocity of a leaky SAW with respect to thethickness of the tantalum electrode. FIGS. 74 and 75 show that a changein the acoustic velocity of the SAW is much larger when the thickness ofthe tantalum IDT is varied than when the thickness of the SiO₂ film isvaried. Accordingly, it is desirable that the frequency is adjustedbefore the formation of the SiO₂ film. It is desirable that thefrequency is adjusted after a tantalum IDT is formed by laser etching orion etching.

In this preferred embodiment, as described above, a 14°-58°-rotatedY-plate X-propagating LiTaO₃ substrate having Euler angles (0°,104°-148°, 0°), a tantalum IDT having a thickness H/λ of about 0.004 toabout 0.055, and a SiO₂ film having a thickness Hs/λ of about 0.10 toabout 0.40 are used. The number and the structure of IDTs are notparticularly restricted. That is, the present invention can be appliedto, not only the SAW apparatus shown in FIG. 15, but also various typesof SAW resonators and SAW filters as long as the above-describedconditions are satisfied.

Euler angle Θ may sometimes deviate from the desired angle by about −2°to about +4°. This deviation is caused by the fact that calculationswere made in this preferred embodiment assuming that a metallic film wasformed on the entire surface of the substrate, and there may be someerrors within the above range in actual SAW apparatuses.

When manufacturing SAW apparatuses, although φ and ψ of the Euler anglesdeviate from 0° by ±3°, substantially the same characteristic as thatwhen φ and ψ are 0° can be obtained.

A SAW apparatus of a seventh preferred embodiment of the presentinvention is described below. The SAW apparatus of the seventh preferredembodiment of the present invention is similar to the SAW apparatus 21of the second preferred embodiment of the present invention shown inFIG. 15. However, as the piezoelectric substrate 22, a 0°-79°-rotatedY-plate X-propagating LiTaO₃ substrate having Euler angles (0°,90°-169°, 0°) was used, and IDTs made of platinum having a thickness H/λof about 0.005 to about 0.054 were used.

In the seventh preferred embodiment, because the 0°-79°-rotated Y-plateX-propagating LiTaO₃ substrate 22 having Euler angles (0°, 90°-169°,0°), the IDTs 23 a and 23 b made of platinum having a thickness H/λ ofabout 0.005 to about 0.054, and the SiO₂ film 25 having a thickness Hs/λof about 0.10 to about 0.40 were used, it is possible to provide a SAWapparatus that has an improved TCF, a large electromechanical couplingcoefficient KSAW, and a small propagation loss. The seventh preferredembodiment of the present invention is described in detail below by wayof a specific example.

FIGS. 76 and 77 illustrate a change in the attenuation constant α whenplatinum IDTs having different thickness values and SiO₂ films havingdifferent thickness values were formed on a LiTaO₃ substrate havingEuler angles (0°, 125°, 0°) and a LiTaO₃ substrate having Euler angles(0°, 140°, 0°).

As is seen from FIG. 76, when Euler angle Θ is 125°, the attenuationconstant α is small when the normalized thickness Hs/λ of the SiO₂ filmis about 0.1 to about 0.4 and when the normalized thickness H/λ of theplatinum electrode is about 0.005 to about 0.06. In contrast, as is seenfrom FIG. 77, when Θ is 140°, the attenuation constant α is large whenthe normalized thickness H/λ of the platinum electrode is about 0.005 toabout 0.06 regardless of the thickness Hs/λ of the SiO₂ film.

That is, in order to decrease the absolute value of the TCF, to achievea large electromechanical coupling coefficient, and to decrease theattenuation constant, the Euler angles of the LiTaO₃ substrate, thethickness of the SiO₂ film, and the thickness of the platinum electrodemust be considered.

FIGS. 78 and 83 illustrate relationships between the attenuationconstant α and Euler angle Θ when the normalized thickness Hs/λ of theSiO₂ film and the normalized thickness H/λ of the platinum electrodewere changed.

As is seen from FIGS. 78 through 83, it is desirable that Θ is from 90°to 169° when the thickness H/λ of the platinum electrode is about 0.005to about 0.054. Combinations of the normalized thickness Hs/λ of theSiO₂ film and Euler angle Θ that reduce the attenuation constant α whenthe normalized thickness H/λ of the platinum electrode is from about0.01 to about 0.04, more preferably from about 0.013 to about 0.033, canbe selected from the combinations shown in Table 12 and Table 13,respectively. The Euler angles of LiTaO₃ shown in Table 12 and Table 13were selected so that the attenuation constant α becomes about 0.05 orlower. The more preferable Euler angles shown in Table 12 and Table 13were selected so that the attenuation constant α becomes about 0.025 orlower. Euler angle Θ shown in Table 12 and Table 13 may vary by about−2° to +4° caused by a variation in the electrode finger width of theplatinum electrode or a variation in the single crystal substrate.

When manufacturing SAW apparatuses, although φ and ψ of the Euler anglesdeviate from 0° by ±3°, substantially the same characteristic as thatwhen φ and ψ are 0° can be obtained.

TABLE 12 Platinum thickness H/λ: about 0.01 to about 0.04 Euler anglesof More preferable SiO₂ thickness Hs/λ LiTaO₃ (°) Euler angles (°) 0.10≦ Hs/λ < 0.15 0 ± 3, 90-169, 0 ± 3 0 ± 3, 105-153, 0 ± 3 0.15 ≦ Hs/λ <0.20 0 ± 3, 90-167, 0 ± 3 0 ± 3, 105-152, 0 ± 3 0.20 ≦ Hs/λ < 0.25 0 ±3, 90-167, 0 ± 3 0 ± 3, 107-152, 0 ± 3 0.25 ≦ Hs/λ < 0.30 0 ± 3, 90-164,0 ± 3 0 ± 3, 104-151, 0 ± 3 0.30 ≦ Hs/λ < 0.40 0 ± 3, 90-163, 0 ± 3 0 ±3, 105-150, 0 ± 3

TABLE 13 Platinum thickness H/λ: about 0.013 to about 0.033 Euler anglesof More preferable SiO₂ thickness Hs/λ LiTaO₃ (°) Euler angles (°) 0.10≦ Hs/λ < 0.15 0 ± 3, 106-155, 0 ± 3 0 ± 3, 116.0-147.5, 0 ± 3 0.15 ≦Hs/λ < 0.20 0 ± 3, 104-155, 0 ± 3 0 ± 3, 113.5-150.0, 0 ± 3 0.20 ≦ Hs/λ< 0.25 0 ± 3, 102-155, 0 ± 3 0 ± 3, 111.5-150.0, 0 ± 3 0.25 ≦ Hs/λ <0.30 0 ± 3, 102-154, 0 ± 3 0 ± 3, 35 U.S.C. §112, .0-146.0, 0 ± 3 0.30 ≦Hs/λ < 0.40 0 ± 3, 102-153, 0 ± 3 0 ± 3, 110.0-144.5, 0 ± 3

When the thickness H/λ of the platinum electrode is about 0.01 to about0.04, as indicated in Table 12, the thickness Hs/λ of the SiO₂ film ispreferably 0.1 to 0.4 in order to set the range of the TCF to be betweenabout −20 ppm/° C. and about +20 ppm/° C. In this case, Euler angle Θ ofthe LiTaO₃ substrate is preferably 90° to 169° (corresponding to therotation angle of 0° to 79°), and more preferably, the Euler anglesindicated on the right side of Table 12 are selected according to thethickness Hs/λ of the SiO₂ film.

Similarly, when the thickness H/λ of the platinum electrode is about0.013 to about 0.033, as indicated in Table 13, the thickness Hs/λ ofthe SiO₂ film is preferably about 0.1 to about 0.4 in order to set therange of the TCF to be between about −20 ppm/° C. and about +20 ppm/° C.In this case, Euler angle Θ of the LiTaO₃ substrate is preferably 102°to 155°, and more preferably, the Euler angles shown on the right sideof Table 13 are selected according to the thickness of the SiO₂ film.

The relationships between the Hs/λ of the SiO₂ film and the Euler anglesshown in Table 12 and Table 13 when the thickness H/λ of the platinumelectrode is from about 0.013 to about 0.033 were determined in terms ofthe thickness H/λ of the platinum electrode shown in FIGS. 78 through83.

When manufacturing the SAW apparatus of this preferred embodiment, it ispreferable that an IDT primarily including platinum is formed on arotated Y-plate X-propagating LiTaO₃ substrate. Then, the frequency isadjusted. Then, a SiO₂ film having a thickness that can reduce theattenuation constant α is formed. This is explained below with referenceto FIGS. 84 and 85. Platinum IDTs having different thickness values andSiO₂ films having different thickness values were formed on a rotatedY-plate X-propagating LiTaO₃ substrate (Euler angles (0°, 126°, 0°)).FIG. 84 illustrates a change in the acoustic velocity of a leaky SAWwith respect to the thickness of the SiO₂ film. FIG. 85 illustrates achange in the acoustic velocity of a leaky SAW with respect to thethickness of the platinum electrode. FIGS. 84 and 85 show that a changein the acoustic velocity of the SAW is much larger when the thickness ofthe platinum IDT is varied than when the thickness of the SiO₂ film isvaried. Accordingly, it is desirable that the frequency is adjustedbefore the formation of the SiO₂ film. It is desirable that thefrequency is adjusted after a platinum IDT is formed by laser etching orion etching.

In this preferred embodiment, a 0°-79°-rotated Y-plate X-propagatingLiTaO₃ substrate having Euler angles (0°, 90°-169°, 0°), a platinum IDThaving a thickness H/λ of about 0.005 to about 0.054, and a SiO₂ filmhaving a thickness Hs/λ of about 0.10 to about 0.40 are used. The numberand the structure of IDTs are not particularly restricted. That is, thepresent invention can be applied to, not only the SAW apparatus shown inFIG. 15, but also various types of SAW resonators and SAW filters aslong as the above-described conditions are satisfied.

A SAW apparatus of an eighth preferred embodiment of the presentinvention is described below. The SAW apparatus of the eighth preferredembodiment of the present invention is similar to the SAW apparatus 21of the second preferred embodiment of the present invention shown inFIG. 15. However, as the piezoelectric substrate 22, a 14°-50°-rotatedY-plate X-propagating LiTaO₃ substrate having Euler angles (0°,104°-140°, 0°) was used, and electrodes made of nickel (Ni) ormolybdenum (Mo) were used.

The IDTs 23 a and 23 b and the reflectors 24 a and 24 b are made of ametal having a density of about 8700 to about 10300 kg/m3, a Young'smodulus of about 1.8×1011 to about 4×1011N/m², and a transversal-waveacoustic velocity of about 3170 to about 3290 m/s. Such a metal includesnickel, molybdenum, or an alloy primarily including nickel ormolybdenum. The normalized thickness H/λ of the IDTs 23 a and 23 branges from about 0.008 to about 0.06.

In the eighth preferred embodiment of the present invention, since the14°-50°-rotated Y-plate X-propagating LiTaO₃ substrate 22 having Eulerangles (0°, 104°-140°, 0°), the IDTs 23 a and 23 b made of theabove-described type of metal having a normalized thickness H/λ of about0.008 to about 0.06, and the SiO₂ film 25 having a normalized thicknessHs/λ of about 0.10 to about 0.40 were used, it is possible to provide aSAW apparatus that has an improved TCF, a large electromechanicalcoupling coefficient KSAW, and a small propagation loss. The eighthpreferred embodiment of the present invention is described in detailbelow by way of a specific example.

FIGS. 86 and 87 illustrate a change in the attenuation constant α whennickel IDTs having different thickness values and SiO₂ films havingdifferent thickness values were formed on a LiTaO₃ substrate havingEuler angles (0°, 120°, 0°) and a LiTaO₃ substrate having Euler angles(0°, 140°, 0°).

As is seen from FIG. 86, when Euler angle Θ is about 120°, theattenuation constant α is small when the normalized thickness Hs/λ ofthe SiO₂ film is about 0.1 to about 0.4 and when the normalizedthickness H/λ of the nickel electrode is about 0.008 to about 0.08. Incontrast, as is seen from FIG. 87, when Θ is about 140°, the attenuationconstant α is large when the thickness H/λ of the nickel electrode isabout 0.008 to about 0.08 regardless of the normalized thickness Hs/λ ofthe SiO₂ film.

FIGS. 88 and 89 illustrate a change in the attenuation constant α whenmolybdenum IDTs having different thickness values and SiO₂ films havingdifferent thickness values were formed on a LiTaO₃ substrate havingEuler angles (0°, 120°, 0°) and a LiTaO₃ substrate having Euler angles(0°, 140°, 0°).

As is seen from FIG. 88, when Euler angle Θ is about 120°, theattenuation constant α is small when the normalized thickness Hs/λ ofthe SiO₂ film is about 0.1 to about 0.4 and when the normalizedthickness H/λ of the molybdenum electrode is about 0.008 to about 0.08.In contrast, as is seen from FIG. 89, when Euler angle Θ is about 140°,the attenuation constant α is large when the thickness H/λ of themolybdenum electrode is about 0.008 to about 0.08 regardless of thenormalized thickness Hs/λ of the SiO₂ film.

That is, in order to decrease the absolute value of the TCF, to achievea large electromechanical coupling coefficient, and to decrease theattenuation constant, the Euler angles of the LiTaO₃ substrate, thethickness of the SiO₂ film, and the thickness of a metal having theabove-described density, the Young's modulus, and the transversal-waveacoustic velocity must be considered.

FIGS. 90 through 93 illustrate the relationships between the attenuationconstant α and Euler angle Θ when the normalized thickness Hs/λ of theSiO₂ film and the normalized thickness H/λ of the nickel electrode arechanged.

FIGS. 94 through 97 illustrate the relationships between the attenuationconstant α and Euler angle Θ when the normalized thickness Hs/λ of theSiO₂ film and the normalized thickness H/λ of the molybdenum electrodeare changed.

As is seen from FIGS. 90 through 97, optimal combinations of thenormalized thickness Hs/λ of the SiO₂ film and Euler angle Θ when thenormalized thickness H/λ of the nickel or molybdenum electrode is about0.008 to about 0.06, about 0.017 to about 0.06, and about 0.023 to about0.06 are shown in Table 14. Euler angle Θ shown in Table 14 may vary byabout −2° to about +4° caused by a variation in the electrode fingerwidth or a variation in the single crystal substrate.

When manufacturing SAW apparatuses, although φ and ψ of the Euler anglesdeviate from 0° by about ±3°, substantially the same characteristic asthat when φ and ψ are 0° are obtained.

TABLE 14 SiO₂ thickness Euler angles of LiTaO₃ More preferable Hs/λ (°)Euler angles (°) 0.1-0.2 0 ± 3, 105-140, 0 ± 3 0 ± 3, 110-135, 0 ± 30.2-0.3 0 ± 3, 105-140, 0 ± 3 0 ± 3, 108-135, 0 ± 3 0.3-0.4 0 ± 3,104-139, 0 ± 3 0 ± 3, 108-133, 0 ± 3

Optimal combinations of the normalized thickness Hs/λ of the SiO₂ filmand Euler angle Θ when the normalized thickness H/λ of the nickelelectrode is about 0.008 to about 0.06, about 0.02 to about 0.06, andabout 0.027 to about 0.06 shown in FIGS. 90 through 93 are shown inTable 15.

TABLE 15 SiO₂ thickness Euler angles of More preferable Hs/λ LiTaO₃ (°)Euler angles (°) 0.1-0.2 0 ± 3, 106-140, 0 ± 3 0 ± 3, 110-135, 0 ± 30.2-0.3 0 ± 3, 105-137, 0 ± 3 0 ± 3, 108-134, 0 ± 3 0.3-0.4 0 ± 3,104-133, 0 ± 3 0 ± 3, 108-132, 0 ± 3

Optimal combinations of the normalized thickness Hs/λ of the SiO₂ filmand Euler angle Θ when the normalized thickness H/λ of the molybdenumelectrode is about 0.008 to about 0.06, about 0.017 to about 0.06, andabout 0.023 to about 0.06 shown in FIGS. 94 through 97 are shown inTable 16.

TABLE 16 SiO₂ thickness Euler angles of More preferable Hs/λ LiTaO₃ (°)Euler angles (°) 0.1-0.2 0 ± 3, 107-141, 0 ± 3 0 ± 3, 110-135, 0 ± 30.2-0.3 0 ± 3, 104-141, 0 ± 3 0 ± 3, 109-135, 0 ± 3 0.3-0.4 0 ± 3,104-138, 0 ± 3 0 ± 3, 108-133, 0 ± 3

When the normalized thickness H/λ of the electrode made of a metalhaving the above-described density, Young's modulus, andtransversal-wave sonic wave is about 0.008 to about 0.06, about 0.017 toabout 0.06, and about 0.023 to about 0.06, as indicated in Table 14, thenormalized thickness Hs/λ of the SiO₂ film is preferably about 0.1 toabout 0.4 in order to set the range of the TCF to be between about −20ppm/° C. and about +20 ppm/° C. In this case, Euler angle Θ of theLiTaO₃ substrate is preferably between about 104° to about 140°(corresponding to the rotation angle of about 14° to about 50°), andmore preferably, the Euler angles shown on the right side of Table 14are selected according to the normalized thickness Hs/λ of the SiO₂film.

Similarly, when the normalized thickness H/λ of the nickel electrode isabout 0.008 to about 0.06, about 0.02 to about 0.06, and about 0.027 toabout 0.06, the normalized thickness Hs/λ of the SiO₂ film is preferablyabout 0.1 to about 0.4 in order to set the range of the TCF to bebetween about −20 ppm/° C. and about +20 ppm/° C. In this case, Eulerangle Θ of the LiTaO₃ substrate is preferably between about 104° toabout 140°, and more preferably, the Euler angles shown on the rightside of Table 15 are selected according to the normalized thickness Hs/λof the SiO₂ film.

Similarly, when the normalized thickness H/λ of the molybdenum electrodeis about 0.008 to about 0.06, about 0.02 to about 0.06, and about 0.027to about 0.06, the thickness Hs/λ of the SiO₂ film is preferably about0.1 to about 0.4 in order to set the range of the TCF to be betweenabout −20 ppm/° C. and about +20 ppm/° C. In this case, Euler angle Θ ofthe LiTaO₃ substrate preferably between about 104° to about 141°, andmore preferably, the Euler angles shown on the right side of Table 16are selected according to the normalized thickness of the SiO₂ film.

The Euler angles of LiTaO₃ shown in Table 14 through Table 16 wereselected so that the attenuation constant becomes about 0.1 or lower.The more preferable Euler angles shown in Table 14 through Table 16 wereselected so that the attenuation constant becomes about 0.05 or lower.The relationships between the Hs/λ of the SiO₂ film and the Euler anglesshown in Table 14 through Table 16 when the normalized thickness H/λ ofthe electrode is from about 0.095, about 0.017, and about 0.023 weredetermined in terms of the normalized thickness H/λ of the nickel ormolybdenum electrode shown in FIGS. 90 through 97.

When manufacturing the SAW apparatus of this preferred embodiment, it ispreferable that an IDT made of the above-described specific metal, suchas nickel or molybdenum, is formed on a rotated Y-plate X-propagatingLiTaO₃ substrate. Then, the frequency is adjusted. Then, a SiO₂ filmhaving a thickness that can reduce the attenuation constant α is formed.This is explained below with reference to FIGS. 98 through 101. Nickeland molybdenum IDTs having different thickness values and SiO₂ filmshaving different thickness values were formed on a rotated Y-plateX-propagating LiTaO₃ substrate (Euler angles (0°, 126°, 0°)). FIGS. 98and 100 illustrate a change in the acoustic velocity of a leaky SAW withrespect to the thickness of the nickel electrode and the thickness ofthe molybdenum electrode, respectively. FIGS. 99 and 101 illustrate achange in the acoustic velocity of a leaky SAW with respect to thethickness of the SiO₂ film. By comparing FIGS. 98 and 99, and FIGS. 100and 101, it is seen that a change in the acoustic velocity of the SAW ismuch larger when the thickness of the electrode is varied than when thethickness of the SiO₂ film is varied. Accordingly, it is desirable thatthe frequency is adjusted before the formation of the SiO₂ film. It isdesirable that the frequency is adjusted after a nickel or molybdenumIDT is formed by laser etching or ion etching.

In this embodiment, a 14°-50°-rotated Y-plate X-propagating LiTaO₃substrate having Euler angles (0°, 104°-140°, 0°), an IDT made of ametal having the above-described density, the Young's modulus, and thetransversal-wave acoustic velocity, such as nickel or molybdenum, havinga normalized thickness H/λ of about 0.008 to about 0.06, and a SiO₂ filmhaving a normalized thickness Hs/λ of about 0.10 to about 0.40 arepreferably used. The number and the structure of IDTs are notparticularly restricted. That is, the present invention can be appliedto, not only the SAW apparatus shown in FIG. 15, but also various typesof SAW resonators and SAW filters as long as the above-describedconditions are satisfied.

While preferred embodiments of the invention have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the invention. The scope of the invention, therefore, is to bedetermined solely by the following claims.

1. A manufacturing method for a surface acoustic wave apparatus, comprising the steps of: preparing a piezoelectric substrate; forming a first insulating layer on the entirety of one surface of the piezoelectric substrate; forming a resist layer directly on said first insulation layer, said resist layer having a resist pattern for forming an electrode pattern including at least one electrode; removing, by using the resist pattern, at least a portion of the first insulating layer in an area in which said at least one electrode is to be formed directly on said piezoelectric substrate; maintaining a laminated structure of the first insulating layer and the resist pattern in an area other than the area in which said at least one electrode is to be formed; forming said at least one electrode by forming an electrode film directly on said piezoelectric substrate including at least one of a metal having a density higher than Aluminum and an alloy including a metal having a density higher than Aluminum in the area of the portion of the first insulating layer which was removed and said at least one electrode having a thickness substantially equal to a thickness of the first insulating layer while maintaining said laminated structure; removing the resist pattern on the first insulating layer; and forming a second insulating layer to cover and be in direct contact with the first insulating layer and said at least one electrode thereby producing said surface acoustic wave apparatus.
 2. The manufacturing method according to claim 1, wherein the density of the metal or the alloy of said at least one electrode is about 1.5 times or greater than the density of the first insulating layer.
 3. The manufacturing method according to claim 1, wherein the metal of said at least one electrode is selected from the group consisting of Au, Cu, Ag, W, Ta, Pt, Ni, Mo, the alloy primarily includes at least two metals from the group consisting of Au, Cu, Ag, W, Ta, Pt, Ni, Mo, and the first and second insulating layers include a layer of SiO2. 